Skip to main content

Full text of "Elements of physical geography"

See other formats


IvIIBRARY 

OF THE 

University of California. 



Class 



Digitized by the Internet Arciiive 

in 2008 with funding from 

IVIicrosoft Corporation 



http://www.archive.org/details/elementsofphysicOOhopkrich 



ELEMENTS OF PHYSICAL 
GEOGRAPHY 






OF 

calif< 




Grand Canyon of the North Platte River, Central Wyoming. (U. G. Cornell.) 



Elements of Physical 
Geography 



BY 
THOMAS CRAMER HOPKINS, Ph. D 

Professor of Geology in Syracuse University 



ov troXX' dWa iroXv 




BENJ. H. SANBORN & CO. 

BOSTON NEW YORK CHICAGO 



G^ 



S^ 



VH^ 



GENERAL 

/ 1 Copyright, 1908 
By raOMAS CRAMER HOPKINS 



»HK MASON-HBNRT PRK88 
STRACU8E. NBW YORK 



PREFACE 

There are good text-books on physical geography, but 
there are many teachers and school departments not satis- 
fied with any of them. The author has endeavored to meet 
the requirements of these teachers as far as such needs could 
be ascertained. With a subject as broad as physical geog- 
raphy there will always be lack of uniformity in the man- 
ner of presentation, as well as in the subject matter. The 
subject is one which is undergoing many changes, and it is 
possible that both the teachers and the subject may be 
ahead of present text-books in many particulars. 

This book is not an experiment. To accommodate the 
many students who were going out to teach in the schools 
of the State, the author, several years ago, attempted to 
bring his courses in physical geography in Syracuse Uni- 
versity into harmony with the requirements of the Educa- 
tional Department of the State. It was then that he 
realized the force of the complaints of many teachers that 
none of the text-books met these requirements. After try- 
ing two of the leading text-books, he abandoned both and 
prepared a text which has been used successfully, in manu- 
script form, for two years in his own classes. Before 
putting it in book form he studied the conditions in the 
public schools of New York and in other states and has 
attempted to prepare a book to meet the needs of teachers 
and the Educational Departments in this country, not with 
the expectation of pleasing all, but with the confident hope 
of meeting the needs of many of those who are dissatisfied 
with the present books. 

It is not important that any class should pursue the 



188509 



vi PREFACE 

subject in the order in which it is presented in this text. 
The author's custom is to begin with Chapter II, because 
his classes commence in September. This season is favora-. 
ble for field work, which has to do more with the contents 
of Chapters II, III and IV than with Chapter I. If the 
work should begin in midwinter, or there should be no field 
work, then the order given might well be followed. It is 
expected, however, that each teacher will follow his own 
plan,— the subject matter is divided into chapters for that 
purpose. Each teacher will naturally expand that part of 
the subject best illustrated by the geographic conditions in 
the proximity of the school. Those in the glaciated area, 
by use of the references can devote more time to the study 
of glacial phenomena. Those on the shore of the ocean, or 
a "large lake, can give more time to shore features. Those 
in the interior can dwell more on the work of streams and 
ground water. Intensify the portion which the pupil can 
best study from Nature. 

Every class in physical geography should have more or 
less laboratory and field work associated with the text-book. 
One of the functions of the text-book— not the only one, by 
any means— is to serve as a handbook, which the pupil 
studies as an aid in the interpretation of what he sees in 
the laboratory. 

To aid the teacher in his work, the author has prepared 
a small laboratory manual to accompany this text. The 
manual must, of necessity, be a book of suggestions rather 
than directions. The work, to be successful, depends on 
the skill and tact of the teacher in getting the pupils to 
study and work wfth real things rather than words about 
them. Yet the author believes that a good book is as much 
needed and fully as important in the work of the laboratory 
as in that of the class-room. Many teachers have not had 



PREFACE vii 

the opportunity to develop a systematic course of labora- 
tory work, consequently much time has been wasted by the 
pupils in routine work. The laboratory manual aims to 
help the teacher and pupil, by suggestions and questions, 
to a knowledge of the earth features and relations. 

The text and manual together aim to assist both teacher 
and pupil into the spirit of one of the most inspiring sub- 
jects in our schools; to bring the pupil into contact with 
Nature in such a way that he may see and realize his own 
position in this w^orld of complex activities, so that by 
observing more closely the familiar phenomena surround- 
ing him in his daily life he may extend his observations 
and knowledge through the less known into the unkpown, 
and thus be an intelligent part of the great world in which 
he lives. 

The author is indebted to many teachers of, physical 
geography in different states in the preparation of this 
text. After the manuscript was written H was submitted 
to a number of prominent teachers in high schools, acad- 
emies and colleges for criticism, and the valuable sug- 
gestions made by them are incorporated as far as possible. 
Especially does he desire to express his indebtedness to the 
following eminent teachers for valuable aid : Professor C. 
E. Peet, Lewis Institute, Chicago; Miss Mary G. Sullivan, 
Buffalo High School; Dr. C. H. Richardson, the author's 
colleague in Syracuse University; Miss Jennie T. Martin, 
City Schools, Washington, D. C. ; Professor James H. 
Smith, Chicago High School; Dr. F. H. H. Calhoun, 
Clemson College, S. C. ; Sarah Emerson Green, formerly 
the author's assistant at Syracuse University; and P. F. 
Schneider of Syracuse. The first three above named read 
both the manuscript and the proof with painstaking care, 
and the others gave valuable aid in reading either the 



viii PREFACE 

proof or the manuscript. Dr. H. A. Peck, Professor of 
Astronomy, gave many valuable suggestions on Chapter I, 
and Morgan R. Sanford, local forecaster for the U. S. 
Weather Bureau, did the same in Chapter X. 

For the photographs illustrating the text the author is 
deeply indebted to many friends and colleagues who are 
credited elsewhere. Special thanks are due to the U. S. Geo- 
logical Survey, the U. S. Fish Commission, the Maryland 
and Vermont State Geological Surveys, and the American 
Museum of Natural History. Where not otherwise credited 
the photographs are by the author, except a very few 
where the photographer is not known. The illustrations 
and explanations of the same form a very important part 
of the text and should be studied as carefully as the 
words. In some instances the picture illustrates the text, 
in others the text is an explanation of a principle best 
learned from the picture. 

T. C. H. 

Syracuse University, 
May, 1908, 



CONTENTS 



CHAPTER 



PAGE 



I. The Earth as a Planet 1 

II. Groundwater and Rivers ......... o ...... . 40 

III. Lakes, Swamps and Waterfalls 100 

IV. Glaciers = 138 

V. The Ocean 169 

VI. Shore Lines 197 

VII. The Land— Minerals, Rocks and Soils. ..... 235 

VIII. Physiographic Agencies 274 

IX. Physiographic Features .................< 309 

X. The Atmosphere ...... 348 

XL Geography of Plants, Animals and Man .... 400 

XII. Physiographic Regions of the United States 449 

Appendix , 473 




CHAPTER I 

THE EARTH AS A PLANET 

Introductory.— Physical geography literally means a 
description of the natural features of the earth. The de- 
velopment of the subject during recent years has led to the 
subdivision as follows : 

1. The earth as a globe or planet, its origin and rela- 
tion to the other heavenly bodies. 

2. The atmosphere or the surrounding gaseous portion. 

3. The hydrosphere or the water, including the fresh 
water and the salt water of the ocean. 

4. The lithosphere or the soli.d land portions, the 
causes producing the various topographic forms and the 
effects of these on climate and life. 

5. Life geography or the effect of physical environ- 
ment upon life and its effect on the earth features. 

Physical geography includes a study of these subjects with 
reference to their influence upon man, his industries, civiliza- 
tion and relation to his surroundings. With this aim in view 
it leads one within the doorway of each of the natural sciences. 

To the ancient philosopher's maxim, "Know thyself," the 
modern scientist adds "in relation to Nature." This is the foun- 
dation of modern physical geography. In gaining this knowl- 
edge man is better able to adapt himself to his surroundings, to 
utilize the various forces and products of Nature, to realize not 
only his dependence upon his fellow man and the lower forms of 
life, but his duty towards them as well, and consciously or un- 
consciously, he must gain respect if not love for the Omnipotent 
Power that rules over all. 

Physical Geography is the science which treats of the 

1 



2 PHYSICAL GEOGRAPHY 

natural features of the earth in their relation to man and 
the lower forms of life. 

1. The Earth a Part of the Solar System.— The earth 
is a nearly round ball consisting of a large rock mass partly 
covered with oceanic waters, and entirely surrounded by 
the gaseous atmosphere. The whole mass solid, liquid and 
gaseous, rotates on its axis as it revolves in space around 
the sun. It is but one of a number of similar bodies called 
planets and is in no wise conspicuous among them. It is 
neither the largest nor the smallest; neither the farthest 
from nor the nearest to the sun. Because we live on the 
earth, it is most important to us, but if we could look on it 
from some distant point in the heavens we should not see 
anything to distinguish it particularly from the other 
planets. 

2. What the Solar System Comprises.— The earth is 
an important member of the solar system which includes 
the sun at the center," the planets and their satellites, the 
planetoids or asteroids, and some comets. Besides the earth 
there are revolving around the sun seven other planets 
which are named in order beginning with the one nearest 
the sun, — Mercury, Venus, Earth, Mars, Jupiter, Saturn, 
Uranus and Neptune. Four of these, Venus, Mars, Jupiter 
and Saturn, are plainly visible at certain periods. Two of 
them, Venus and Jupiter, are at times the brightest bodies 
in the heavens except the sun and moon. Part of the time 
they are morning stars ; at other times evening stars. The 
planets may be distinguished from the true stars by their 
steady light. The stars twinkle. Each of the planets, ex- 
cept Mercury and Venus, has one or more satellites or 
moons revolving around it. Saturn has besides the satel- 
lites several concentric bright rings surrounding it. The 
relative sizes, distances and other data concerning the 
planets are given in Appendix I. 



THE. EARTH AS A PLANET 3 

The asteroids or planetoids, about 600 in number, are solid 
bodies much smaller than the planets, and revolve in orbits be- 
tween Mars and Jupiter. One of the planetoids, Eros, about 20 
miles in diameter, discovered in 1898, has a very eccentric orbit 
that sometimes brings it within 13 1/^ million miles of the earth. 
The student should learn to recognize the larger planets and ob- 
serve their movements among the stars from season to season. 



^llE SOLAR SYSTUA, 




Fia. 1. The solar system, showing the order of the planets, satellites, 
asteroids, and the orbits of a few comets. 

3. Relation of the Solar System to the Universe.— 

The Solar System, large and complex as it appears, is but 



4 PHYSICAL GEOGRAPHY 

one of a number of similar systems in the universe. Most 
of the bright stars in the heavens are suns similar to ours. 
They appear to be much smaller than our sun, but that is 
because they are so much farther away. In reality many 
of them are much larger. They probably have planets, 
satellites, comets, etc., like our own system, but they are so 
far away that these bodies, if they exist, are not visible 
from the earth. It is not known how many of these sys- 
tems there are, nor how far out in space they extend, but 
certainly a great distance beyond our comprehension. It 
is estimated that with a large telescope one can see between 
100 and 200 millions of stars, a large per cent of which lie 
in the Milky Way. With few exceptions all of these stars 
are so far aw^ay that it takes the light from them travelling 
at the rate of 186,000 miles a second many years to reach 
the earth. The moon is about 240,000 miles away or about 
ten times the distance around the earth ; the sun is nearly 
400 times farther than the moon; and the nearest fixed 
star or neighboring sun system is several thousand times 
farther than the sun. The light of the sun takes about 
8 minutes to reach the earth. The light of the nearest star 
takes 31/2 years to cross the space separating it from the 
earth. Truly the earth is a very small part of the solar 
system and an exceedingly minute portion of the universe. 
4. The Moon.— The earth has one satellite, the moon, 
which revolves around it once a month (27.32 days) and 
accompanies it through space in its journey around the sun. 
The moon is 2,163 miles in diameter and at an average 
distance of 238,840 miles from the earth, but it varies from 
221,600 to 252,970 miles. It is because of its nearness to 
the earth that it is held in its orbit around the earth in- 
stead of pursuing an independent course around the sun. 
(The synodic month or the time from full moon to full 
moon is 29.53 days, but the sidereal month is 27.32 days.) 



THE EARTH AS A PLANET 5 

5. The Phases of the Moon.— The moon emits no light of its 
own. All the light that comes from it to the earth is reflected 
sunlight. When the moon is in that part of its orbit nearest the 
sun, it is nearly between the earth and the sun, and we see but 
a mere fringe of illumination; it is then the new moon. The 
sunlight reflected from the earth faintly illuminates its dark side 
giving what is called the earth shine. When it has completed a 
fourth of its circuit after new moon, it is at right angles to a 
line connecting the sun and the earth, and we see one-half of the 
illuminated face, that is, a fourth of the whole surface, and the 
phase is called the iirst quarter. When it has completed half a 
circuit and is on the opposite side of the earth from the sun, it 




Fig. 2. The phases of the moon. 

is full moon. At the third quarter the moon has completed three- 
fourths of its circuit and one-fourth of the whole surface is again 
reflecting light to the earth. The line separating the illuminated 
portion from the dark portion is called the terminator. Draw 
from observation a figure of the moon showing the light and dark 
portion every second night from one new moon to the next; 
arrange them in order around an ellipse and compare them. 

6, The Sun. — The sun is the center of the solar sys- 
tem. All the planets of the system revolve about it and 
receive heat and ligrht from it. It is much larger than all 



6 



PHYSICAL GEOGRAPHY 



the planets combined, having a diameter of 866,000 miles, 
which would make it a million times the bulk of the earth ; 
but since its density is less, it has only 332,000 times the 
mass of the earth. Imagine the earth at the center of the 
sun and the moon revolving around it in an orbit the same 
size as the present one ; the moon would then be about half 
way from the center to the circumference of the sun. 
7. The Sun's Energy.— Nearly all the heat, light, and 
other forms of energy on the surface of the earth come 
directly or indirectly from the sun. The radiant energy 

from the sun, known as 
insolation, is thought to 
pass from the sun to the 
earth unaffected by in- 
tervening space until it 
reaches the earth where 
part of it, the part that 
we recognize, is percep- 
tible as heat and light. 
The part of the sun's 
insolation received by 
the earth is an exceed- 
ingly small part of the 
whole, and when one 
realizes that- nearly all 
forms of heat and light come from the sun, the total 
quantity radiated into space is something beyond compre- 
hension. All the energy used by man in heating and light- 
ing, all that is used in running machinery everywhere, -11 
that is used in lifting the waters of the sea to the clouds to 
fall as rain, all that wonderful vital energy manifested in 
animals and plants,— all of these and probably other forms 
of energy as yet unrecognized are flashed like wireless tele- 
grams across the vast space that separates us from the sun. 




Fig. 3. Showing the relative size of the 
sun and the moon's orbit. What is 
the scale of the diagram? 



THE EARTH AS A PLANET 



8. Eclipses.— Since the sun is the source of the light 
received by the earth and the moon, when either of these 




Annular EclipM 

Fig. 4. Solar and lunar eclipses, 

latter bodies comes between the other and the sun, the 
light of the sun will be cut off. The shadow thrown by 



8 PHYSICAL GEOGRAPHY 

the intervening body on the other is known as an eclipse. 
The shadow of the moon on the earth produces an eclipse 
of the sun, and the shadow of the earth on the moon causes 
an eclipse of the moon. If the moon passes entirely into 
the earth's shadow, there is a total eclipse of the moon, if 
only part of it passes into the shadow, a partial eclipse is 
the result. There may be three kinds of solar eclipses : ( 1 ) 
a total eclipse when the moon passes centrally over the disc 
of the sun and so near the earth that the shadow reaches 
the earth; (2) an annular eclipse, produced when the moon 
passes centrally over the disc of the sun but is so far from 
the earth that the end of the shadow does not reach the 
earth ; then the moon appears as a black spot in the center 
of the sun surrounded by a ring of light which gives the 
name annular or ring eclipse ; ( 3 ) a partial eclipse of the 
sun produced when the moon passes a little to one side of 
the line joining the earth and the center of the sun. 

If the moon revolved about the earth in the plane of the 
earth's orbit, there would be a total eclipse of the moon and sun 
once each month, but since the plane of the moon's orbit is in- 
clined at an angle of five degrees to that of the earth's 
orbit, there is an eclipse only when the moon passes one of the 
nodes, that is, the points of intersection of the two orbits, at or 
near new moon or full moon. There may be an eclipse of the sun 
when there is none of the moon, and there must be at least two 
solar eclipses each year. Consult the almanac for several years 
and see how many eclipses of each kind there have been. 

In 1907 there were four eclipses, two of the sun, one total 
and one annular, and two of the moon. 

9. Comets.— Comets belong in part to the solar sys- 
tem. Several hundred of these bodies have been seen from 
the earth at different times. Some of them travel in ellip- 
tical orbits which extend millions of miles out into space 
beyond the outermost planet in our system, hence the period 
of revolution is one of many years. Many comets travel in 



THE EARTH AS A PLANET 9 

a parabola or a hyperbola and become visible once as they 
pass around the sun and away again, never to return. 
Whence they come and whither they go is not known. 
Prom fig. 5 it can be seen that parabolas and hyperbolas 
are curved lines, the ends of which never meet. 

The comets differ in size and shape as widely as they do 
in their orbits. They are characterized by a nucleus or 
denser portion surrounded by a nebulous mass called the 
coma which streams out from the nucleus and forms the 
tail. The tail is single or double and of widely diverse 




Fig. 5. Ellipse, parabola and hyperbola. The last two are diverging curves 
which never meet. 

shapes and differs in length from that of the diameter of 
the nucleus to a length of 100,000,000 miles or more. As 
a comet approaches the sun, the tail streams out behind it, 
as it passes perihelion (the nearest point to the sun), the 
tail streams out ahead of it, that is, the tail keeps on the 
opposite side of the nucleus from the sun. Celestial pho- 
tography has shown recently that the tails of several com- 
ets have been suddenly broken into two or more parts. 

10. Historical Comets.— The comet of 1680 is an important 
one because it was the first whose orbit was determined by the 



10 PHYSICAL GEOGRAPHY 

principles of gravitation. The computation was made by Sir 
Isaac Newton who found that it passed within 140,000 miles of 
the sun travelling at the rate of 1,332,000 miles an hour. It had 
a tail 100,000,000 miles long. 

Ealley's comet (1682) is so called because Halley, a friend of 
Newton, computed its orbit and thus identified it with previous 
comets that had appeared at intervals of 75 years. He predicted 
that it would make its next appearance March 13, 1759. It passed 
perihelion within a month of that time. It appeared in 1835 and 
is due again in 1910. Watch for it. 

Biela's comet (1826) was observed in the latter part of Decem- 
ber, 1846, to elongate and divide into two parts which travelled 
in parallel orbits 160,000 miles apart. When they next appeared 
in 1852 the two portions were 1,500,000 miles apart. They have 
not been seen since. 

The Comet of 1882 was the most conspicuous one in recent 
years. It approached the sun in perihelion close enough to pass 
through part of its gaseous envelope. Daniel's comet attracted 
attention in the summer of 1907. 

11. Meteors and Shooting Stars.— Meteors and shoot- 
ing stars are luminous bodies which are frequently observed 
in our upper atmosphere and are sometimes seen to strike 
the earth. The luminosity of these bodies is thought to be 
due to friction against the atmosphere and that before 
entering the atmosphere they are cold and non-luminous. 
Many of them are dissipated in the upper atmosphere, but 
probably the fragments in the form of invisible dust reach 
the earth in the course of time. Ten to twenty millions of 
meteors strike the earth's atmosphere every day. It is 
thought by some that the earth has been formed by the 
aggregation of such particles, which would mean that un- 
less the earth is losing matter in some way it is still increas- 
ing in weight. 

12. Meteorites.— Meteors which fall to the earth are 
called meteorites. They vary in size from very minute 
fragments to bodies of many tons in weight. The great 



THE EARTH AS A PLANET 11 

Tent meteorite in New York City which Peary brought 
from Cape York, Greenland, weighs 36.5 tons. The 
Bacubirito meteorite in Mexico weighs about 27.5 tons. 
The Willamette meteorite, shown in fig. 6, weighs 15.6 tons. 
Some are composed of stone, some of metals and some of 
both. About four out of every hundred are nearly pure 
iron with a little nickel. The source of meteors and 
meteorites is not definitely known. 




Fig. 6. Willamette meteorite, the third largest known, found 
near Oregon City, Oregon. Length 10 ft., height 6 ft. 6 in., 
weight 15.6 tons. (American Museum of Natural History.) 

THE ORIGIN OF THE EARTH 

All material things so far as we know have a beginning, 
a period of growth, decline, and death. This is not true 
of matter itself but of the forms which it takes. The fact 
is commonly recognized in regard to plants and animals 
but is probably no less true of many inanimate objects, ex- 
cept that the changes" go on so much more slowly that they 
are frequently not recognized. It is now known that the 



12 PHYSICAL GEOGRAPHY 

hills are not *' everlasting. " They may be ''rock-ribbed" 
but they are not as ''ancient as the sun." The mountains 
have a beginning, and a period of growth, after which they 
begin to dwindle and gradually disappear. So it is with 
the earth, the sun, and the solar system ; they did not al- 
ways exist as such. When and how were they formed ? 

13. The Nebular Hypothesis.— Of the many attempts 
to explain the origin of the solar system none has met with 
more favor than that known as the nebular hypothesis, 
which assumes that at one time all the material in the solar 
system existed in the form of a rotating mass of nebulous 
gas that occupied all the space from the center of the 
present sun out to and beyond the limits of the orbit of the 
outermost planet— Neptune. Under the universal law of 
gravitation, by which every particle of matter in the uni- 
verse attracts every other particle, these gas particles were 
attracted towards a common center. In the course of time 
a portion of the mass was separated in the form of a ring, 
or, as some say, as an irregular mass, which in time, by its 
rotation on its own axis, formed a spheroidal body revolv- 
ing around the central mass. This was the planet Neptune, 
which continued to revolve around the central mass from 
which in turn the other planets and their satellites were 
separated, the earth being the sixth one and Mercury the 
last one. The residual central mass is the sun, which, ac- 
cording to the hypothesis, is still contracting. The plan- 
etary masses probably separated from the parent mass 
while still in the gaseous condition, but continued to con- 
tract until they became liquid and on further cooling, solid, 
at least on the surface. 

This hypothesis, with sundry modifications, has been widely 
accepted because it seemed to account for so many things about 
the system. Recently many objections to this explanation have 



THE EARTH AS A PLANET 13 

been raised, while another explanation has been growing in favor 
with some people because it appears to be free from some of the 
difficulties in the nebular hypothesis. 

14. The Planetesimal Hypothesis.*— The planetesimal 
hypothesis, although it starts with a nebulous mass, differs 
radically from the nebular hypothesis in a number of 
particulars. Unlike the first, however, the nebula is not 
necessarily a gas nor is it highly heated and hence it need 




Fig. 7. A spiral nebula in Ursa Major. (Ritchey, Yerkes Observatory) 



not pass through a liquid state. The hypothesis starts with 
a spiral nebula, which is one of the most common forms in 
the sky at present. The knots or denser portions in the 
nebula are the nuclei of the future planets and satellites, 

* Formulated by Professors Chamberlin and Moulton of the University 
of Chicago. 



14 



PHYSICAL GEOGRAPHY 



and the nebulous haze surrounding the nuclei consists of 
finely divided matter mostly solid, possibly some liquid and 
gaseous, which is later gathered in by gravitation, and added 
to the nuclei to form the planets. All of the material, first 
in the nebula and later in the planets and satellites, moves 
about the central mass in elliptical orbits. It all has a 
double motion, first around the central axis of its own 
nucleus or planet, and second around the central sun. 




• Fig. 8. The great nebula in Andromeda. (Ritchey, Yerkes Observatory.) 



The hypothesis supposes a relatively slow growth of the 
earth, as of the other planets, with increasing temperature 
in the central portions due to gravity. That is, a large 
body will have greater pressure by gravity at the center 
than a small one and, hence, will have greater heat induced 



THE EARTH AS A PLANET 15 

by the pressure. In a body as large as the earth, the gravi- 
tative attraction of all the particles towards the center pro- 
duces an enormous pressure on the central portions, a pres- 
sure sufficient to produce heat and raise the temperature 
of the interior. 

For a long time after the earth nuclues began to grow 
it was too small to have an atmosphere or even a hydro- 
sphere, both of which formed gradually as soon as the 
planet was large enough to hold them by the force of 
gravity. They would be increased by the parts expelled 
from the interior by gravity pressure, as well as the parts 
that would be drawn to the surface of the mass from the 
surrounding nebula. Hence, the accretion of the planet- 
esimal matter of the outer half or more of the earth would 
be through an atmosphere and subject to the action of 
moisture. This hypothesis likewise makes possible a much 
longer period of time in which life was possible on the 
earth or in which the conditions favored the existence of 
life in the initial stages, than does the nebular hypothesis. 

The principal points of difference between the two hypotheses 
are, that according to the first, the earth passes from a highly 
heated gaseous condition through a hot molten state to the pres- 
ent solid condition, while according to the second, the earth was 
never entirely gaseous, never necessarily molten and possibly 
never much hotter than at present. By the first, the earth was 
once larger than at present and included the moon which was 
later separated from it; by the second, the earth was never larger, 
probably not so large in the past as at the present. By the first, 
the outer planet, Neptune, is the oldest and the inner one. Mer- 
cury the youngest; by the second, the planets and their satellites 
are all of about the same age, that is, they were all in process of 
formation at the same time. According to the planetesimal 
hypothesis the moon is devoid of water and an atmosphere be- 
cause it is too small to hold them on the surface by gravity, and 
not because it is so old that it has lost them as sometimes 
claimed in the nebular hypothesis. 



16 PHYSICAL GEOGRAPHY 

15. The Shape of the Earth.— The earth is the shape 
of a ball that is flattened at the poles and bulged at the 
equator, so that the equatorial diameter is nearly 27 miles 
longer than the polar diameter. It approaches an oblate 
spheroid more nearly than any other mathematical figure. 

Evidence that the earth has a curved and not a flat sur- 
face: (1) Its shadow on the moon is always a curved one. 
Could this be true of a flat surface? (2) New stars ap- 
pear in front of the observer and old ones disappear behind 
him as he travels toward the north or south. How would 
• 




Fig. 9. Expansion of horizon with elevation indicates curvature of the earth. 

it be on a flat surface? (3) The horizon expands rapidly 
as the observer ascends to higher altitudes. Would this be 
true on a flat surface? (4) At sea the slender toprigging 
of a vessel is visible farther than the larger but lower hull. « 
Why? (5) There is a marked difference in time with a 
change of longitude; thus the sun rises more than three 
hours later in San Francisco than it does in Philadelphia, 
and nearly nine hours later than it does at London. How 
would it be if the earth were flat? (6) The earth has been 
circumnavigated many times. (7) The flattening at the 
poles is indicated by the increased weight of a body in high 
latitudes over that of the same body at the equator, and by 



THE EARTH AS A PLANET 17 

the greater length of a degree of latitude near the poles. 
(See sec. 28 and fig. 16). 

16. Cause of the Shape of the Earth.— Nearly 200 
years ago it was shown that an oblate spheroid is one of 
the figures of equilibrium for a rotating body, and the de- 
gree of oblateness or flattening is due to the rate of rota- 
tion. More recently it has been shown that the oblateness 
of the earth corresponds to the requirements of a rotating 
fluid mass of the size and rate of rotation of the earth. 
This was cited as evidence that the earth was fluid at one 
time in its history before reaching its present solid form. 
But there are good reasons for thinking that a solid earth 
would take the same shape after a long period of time, due 
to the shifting of materials on the surface, or according to 
the planetesimal hypothesis there would be more material 
accumulate at the equator than at the poles. 

Gravitation shapes the material into a sphere. Rotation 
causes the flattening of the sphere into the oblate spheroid. It 
is gravitation that holds bodies on the earth, and the force in- 
creases with the mass of the planet. If the earth were the size 
of the moon, bodies would have much less weight on its surface. 
Gases would be so light that they would fly off into space and 
there would be no atmosphere, hence no water, and no life. If 
the earth were as large as Jupiter, bodies on the surface would 
be correspondingly heavier. 

17. Size of the Earth.— The diameter of the earth 
through the poles is 7,899.6 miles; through the equator 
7,926.6 miles. Compute the circumference, the area, the 
volume and the weight of the earth in tons from the follow- 
ing data: 

1. The circumference equais the diameter multiplied 
by 3.14159. 

2. The area equals the product of the diameter by the 
circumference of a great circle. 



18 



PHYSICAL GEOGRAPHY 



3. The volume equals the area of the surface multi- 
plied by one-third of the radius. 

4. The mass equals the volume multiplied by the den- 
sity. The mean density of the earth is 5.6. A cubic foot 
of water weighs 62.5 lbs. 

18. Problem of Eratosthenes.— The diameter of the earth 
was not the dimension first determined, as there is no way of 
measuring it directly. The part that was actually measured was 
an arc of the circumference. This problem was first solved by 
Eratosthenes two centuries before the Christian era. He deter- 




FiG. 10. Illustrating the problem of Eratosthenes. The sun's rays, vertical 
at A, are inclined 7° 12' to the vertical sz' at s, which is the angle at 
the center of the earth, C, measured by the arc AS. Fifty times this arc 
equals the circumference of the circle or the distance around the earth. 

mined that Syene in Egypt was close to the same meridian, 
hence to the same great circle as Alexandria. He had observed 
that at noon on the longest day in midsummer the sun's rays 
shone on the bottom of a deep well at Syene in Egypt. ' What 
inference could he draw from this? He measured the angular 
distance of the sun from the zenith at Alexandria on the same 
day at noon and found it equaled 7 degrees and 12 minutes, or 
exactly one-fiftieth of a circle, which is the same as the angle at 



THE EARTH AS A PLANET 19 

the center of the earth formed by the radii from these cities. 
Prove this. The distance between the two cities had been 
measured and found to be 5,000 stadia, hence by multiplying this 
distance by fifty he obtained the total distance around the earth 
as 250,000 stadia. Unfortunately we have no means of knowing 
at the present time the length of a stadium in any of our units 
of measurement, so that we have no certain means of comparing 
the accuracy of the result obtained by Eratosthenes with those 
obtained by similar means in more receot times. 

19. Structure of the Earth.— The earth is frequently 
divided for convenience of study into three parts or spheres : 
(1) The outer gaseous envelope, the atmosphere; (2) the 
liquid envelope, the water or hydrosphere which nearly 
surrounds (3) the solid rocky part, the lithosphere, the 
inner portion of which is sometimes called the centrosphere. 
A fourth is sometimes added called the biosphere, or life 
sphere. These are not true mathematical spheres, nor are 
they very sharply separated at times. Both the hydro- 
sphere and the atmosphere penetrate the lithosphere and 
large quantities of the atmosphere are dissolved in the 
hydrosphere as well as large quantities of water as invis- 
ible vapor in the atmosphere. The life sphere is confined 
chiefly to the water and the contact of the atmosphere 
with the lithosphere. It is scattered through the water 
sphere to a greater depth, probably, than in either the gas- 
eous or the rock portions, yet the greater portion of it lies 
close to the lower portions of the atmosphere. The air, 
water and land portions of the earth, which form the 
greater part of the subject of physical geography or 
physiography, are discussed in the following chapters. 

20. Motions of the Earth.—The earth has (1) a daily 
rotation on its axis, and (2) a yearly revolution around 
the sun. Besides these, it has (3) an onward motion 
through space in company with the other parts of the solar 
system, but this is not so apparent as the other two and is 



^l^ 



20 PHYSICAL GEOGRAPHY 

not marked by such pronounced effects. There are a 
number of other minor motions of interest to the astron- 
omer. 

21. Rotation.— The rotation of the earth on its axis 
causes the sun and the stars to appear to revolve about the 
earth, the sun appearing to rise in the east and set in the 
west, producing the successive changes of day and night 
and thus giving the measure of time, the day. The rota- 
tion of the earth is one of the factors along with others in 
producing the tides, the belts of planetary winds and 
calms; and it affects the direction of the ocean currents. 
The rotation also produces the bulging at the equator and 
the flattening at the poles. It causes a deflection of fall- 
ing bodies. A ball dropped from the top of a tower would 
be deflected to the east of the base of the tower, instead of 
falling directly vertical. The deviation is greatest at the 
equator and zero at the poles. AVhy? In the latitude of 
New York it is about 1 inch for a fall of 500 feet. 

22. Foucault's Pendulum.— In the middle of the last century 
Foucault demonstrated the rotation of the earth by means of a 
pendulum consisting of a heavy weight suspended on a long, 
slender cord which is started to swing due north and south 
across a plane surface covered with fine sand. Attached to the 
bottom of the pendulum is a sharp point which traces a mark in 
the sand as it swings. If the earth were still, the pendulum 
would continue to swing on this line but the rotation causes this 
plane under the pendulum to rotate to an extent varying with 
the latitude, from zero at the equator to a complete revolution at 
the poles. This pendulum is still in use at the Pantheon in Paris 
where visitors may see the rotation taking place as Foucault did 
in 1851. 

23. Directions.— The terms north, south, east and 
west are used to signify directions on the surface of the 
earth and also in space. North with reference to the 
earth really means the direction of the north pole, one end 



THE EARTH AS A PLANET 21 

of the axis of the earth, and would be a curved line cor- 
responding to the meridian at the point where the direc- 
tion is taken. What we really think of, however, is the 
line on the plane of the horizon which marks its intersec- 
tion with the plane of the meridian. North in the heavens 
refers to the direction of the axis of the earth prolonged 
to infinity passing nearly through the north star. At a 
point on the equator this direction would be identical with 
north on the earth, but as one approaches the north pole 
the two lines diverge until near the pole they are at nearly 
right angles to each other. Represent this by a diagram 
for (1) your latitude, (2) the equator and (3) the north 
pole. 

At the north pole all directions on the horizon are south 
and the line to the north star is perpendicular to the hor- 
izon. At all other points on the earth, south is the oppo- 
site direction from north until one arrives at the south 
pole where there is no south but all directions are north. 

East refers to the direction on the horizon at right 
angles to the north and south line but which if followed 
proves to be a curved line. West is the opposite of east. 
The equator and the parallels of latitude are east and 
west lines yet they are circles on the globe. The terms 
east and west are used for directions of rotation and revo- 
lution-, thus the earth rotates toward the east because any 
one point on the earth at any instant is moving east in the 
plane of the horizon. 

The plane of the horizon is the plane perpendicular to 
the plumb line. The point where the extension of the 
plumb line pierces the heavens is called the zenith and the 
direction is up. The point opposite the zenith is the 
nadir and the direction is down. 

24. Revolution.— The revolution of the earth around 
the sun causes the latter to appear to shift its position in 



22 



PHYSICAL GEOGRAPHY 



the heavens from day to day. lu connection with the in- 
clination of the earth's axis the revohition also causes the 
change of seasons. The earth travels around the sun in 
an elliptical orbit with the sun at one focus of the ellipse. 
At the nearest point (perihelion) it is about 91,500,000 
miles distant; at the most remote point (aphelion) it is 
about 94,500,000 miles away. The inclination of the 




Fia. 11. The seasons. The sun is about three million miles nearer the earth 
in January than it is in July. Notice the variation in light and darkness 
through the different seasons. 

earth's axis to that of the axis of the ecliptic or the earth's 
path around the sun is 23 degrees 27 minutes, which means 
that the plane of the earth 's equator is inclined at the same 
angle to that of the plane of the ecliptic. This inclination 
remains fixed (or nearly so) with reference to space and 
distant stars in the heavens, so that the axis of the earth 



THE EARTH AS A PLANET 23 

with slight variations always points to the same star in the 
heavens, but it causes the earth to assume quite different 
positions with reference to the sun, as shown in fig. 11. 

25. The Seasons.— On December 21st the northern 
hemisphere reaches its maximum inclination from the sun, 
the vertical rays of the sun are on the Tropic of Capri- 
corn, their southern limit, and it is then the winter solstice 
(Sun stands). The area around the north pole is in dark- 
ness, and it is winter in the northern hemisphere, while the 
area around the south pole is in continual sunshine and it 
is summer in the southern hemisphere. The opposite con- 
dition prevails on June 21st, the summer solstice, wheni the 
northern hemisphere is inclined toward the sun and the 
rays are vertical on the Tropic of Cancer. It is then 
summer in the northern hemisphere and winter in the 
southern. On March 21st and September 23rd the axis 
of the earth is perpendicular to the line joining the center 
of the earth and the center of the sun and the sunshine 
extends from pole to pole when the days and nights are 
equal (the equinoxes). Name the corresponding seasons. 

By consulting fig. 11 it may be seen that the winter in 
the northern hemisphere does not come when the earth is 
farthest from the sun, but when it is near perihelion, and 
the summer season when it is near aphelion, showing that 
the few degrees difference in the angle at which the sun's 
rays strike the earth have a greater influence on the tem- 
perature than the three millions of miles difference in 
distance. The heat of summer and the cold of winter are 
increased by reason of long days and short nights in the 
summer and long nights and short days in the winter. 

It is thought that the space between the sun and the 
earth is exceedingly cold; and that the sun's rays or inso- 
lation are changed to heat only after entering the earth's 



24 



PHYSICAL GEOGRAPHY 



atmosphere, and very little there until they strike the 
solid earth. 

If the axis of the earth were perpendicular to the plane of 
the ecliptic what would be the effect on the seasons? What 
would be the effect on the winter and summer in New York State 
if the axis were inclined twice as much as at present? The axes 
of some of the other planets are inclined much more than that 
of the earth. (See Appendix I.) 

26. Localization of Places,— Latitude and Longitude. 

— In Altoona, Pa., all the roadways running north and 

south are called avenues 
and those running east and 
west are called streets, and 
both are numbered con- 
secutively. Now if the 
place where the numbering 
begins is known, one only 
needs to know the number 
of the street and avenue 
to locate any place in the 
city with reference to any 
other point. (Study fig. 

12). 

The plan in the above 
city is a modification of 
that used in locating places 
on the globe or on maps 
representing a part of the surface of the globe. Lines on the 
globe represent imaginary ones on the earth and those run- 
ning north and south from pole to pole are called meridians 
of longitude while those running east and west around the 
earth are called parallels of latitude, except the one mid- 
way between the poles which is called the equator. Since 
each one of these lines is a circle around the earth it con- 





.g 




Fjth 


St 
















llh 


St 


■ a 














3rd 


St 












• 




2nd 


St 
















Isl 


?it 






b 










Mai 


^ § 
















O 


v> 


3 


CO 

3- 


^ 
? 


or 












? 


< 


> 


< 








> 










IV — 








a 













Fig. 12. City streets on N-S and 
E-W lines, a is at 4th street and 
2nd avenue N. E. or, from the 
center of the city, a is 2 blocks 
east and 4 north. Locate b, d, 
and g in the same way. 



THE EARTH AS A PLANET 



25 



tains 360 degrees and may conveniently be divided into 
360 parts, each representing one degree. Where more 
lines are desired each degree may be divided into 60 parts 
called minutes and each of these again divided into 60 
parts called seconds and each of these into as many frac- 
tions as desired. Not all of these lines are drawn on the 
globe; in fact only a few of them are represented, but it 
is understood that the space between any two may be sub- 
divided as indicated. It is only necessary to understand 
the method of numbering the lines and the starting point 
to know the location of any point on the earth, when its 

latitude and longitude 
are known. Latitude is 
measured north and 
south from the equator 
to the poles. Since the 
poles are 90 degrees 
from the equator there 

Fig. 13. Latitude and longitude. Lo- ^^n be Only 90 dcgrCCS 
cate points a, b, c, d, e, f, and g the i i • i 

same as in fig. 12 using degrees of north latitude and the 

latitude in place of streets and degrees game number in SOUth 
of longitude in place of avenues. . 

latitude. 

27. Determination of the Latitude.— At sea the latitude is 
generally determined by finding the altitude of the sun by means 
of an instrument called the sextant. There are several different 
ways in which it can be determined on the land without the use 
of a sextant or any other expensive instrument. The north star 
is very nearly vertical over the north pole, hence its altitude 
over that point is 90 degrees. At the equator the north star 
would appear on the horizon, that is, its altitude would be zero. 
Hence the altitude of the north star above the horizon gives the 
latitude of any place in the northern hemisphere, subject to a 
slight correction. For methods of finding the altitude of the 
star, see laboratory exercises. 

At the times of the equinoxes, March 21st and September 
23rd, the sun is on the equator, where a person at noon on the 











"" 






? 












e 


3n' 
















i: 














m° 
















3 










b{ 




















5. 


Eni 


nfni 


















e 








y 














Q' 
















3 










1 




fO* 






4 


"^J 


fe 


»• J- 


>• 


o 

3- 


^'i? 


Pl^ 


P'1 


(T, 






3n' 




















" 














26 



PHYSICAL GEOGRAPHY 



dates mentioned would see the sun directly overhead or at an 
altitude of 90 degrees, and a person at the north pole at the same 
time would see the sun on the horizon. Hence on the dates 
mentioned the altitude of the sun at midday, when subtracted 
from 90 degrees, would give the latitude of the place. On any 
other day in the year, the same method may be followed by the 
subsequent addition or subtraction of the sun's angular distance 



North fble 




Soaa/bb' 



Somhfbh 



MtrtJuuta of L^n^iAuit 

Fig. 14, Parallels and meridians. In the upper figure the lines do not 
meet at the center of the earth because the meridians are not circles; 
the angles are measured by the arc on the surface. 

from the equator. This can be obtained for any day in the year 
by consulting a nautical almanac. In this method care must be 
taken to get the altitude of the sun when it is on the meridian or 
the true noriln and south line, which may be determined by means 
of a magnetic needle or compass and the correction made for the 
local magnetic variation; or at night by noting and marking 



THE EARTH AS A PLANET 



27 



carefully the direction of the north star; or by noting the direc- 
tion of the sun's shortest shadow cast by any vertical post. (See 
laboratory exercises.) 




Fig. 15. Determination of latitude from the north star; hh, hh' etc. plane 
of horizon. At the equator E the star, is in the horizon, elevation and 
latitude zero. At 40 N. latitude elevation of N. star is 40°. Elevation 
of the star at any point equals latitude of the place. 



28. Length of a Degree 
were a perfect sphere the 
degrees of latitude would be 
of the same length in all 
places, but since it is bulged 
at the equator and flattened 
at the poles, the degree is a 
little longer at the poles, 
being 69.407 miles, while on 
the equator it is 68.704 mile». 
(Fig. 16). 

29. Longitude. — Since 
longitude is measured east 
and west around the earth it 
is necessary to select a begin- 
ning point which is called the 
prime meridian. Any merid- 



of Latitude.— If the earth 

90^ -TOO 




Fig. 16. Degrees of latitude are 
longer at the poles than at the 
equator because they are meas- 
ured by the arc of the curve and 
the flattening at the poles makes 
the arc approach that of a larger 
circle than at the equator. The 
differences are exaggerated for 
clearness. 



28 PHYSICAL GEOGRAPHY 

ian might be selected, but the one commonly used by the 
English speaking nations is the one which passes through 
the Royal Observatory at Greenwich, England, and is called 
the meridian of Greenwich. Prom this point the longi- 
tude is counted 180 degrees east and the same number 
west, the two meeting on the 180th meridian. Why are 
there only 90 degrees of latitude and 180 degrees of longi- 
tude ? Why couldn 't the longitude be counted all the way 
around in one direction, 360 degrees either east or west ? 

Degrees of longitude are longest on the equator (69.652 
miles) and grow shorter both north and south from the equator 
to zero at the poles. A degree at 40 degrees latitude equals 
53.431 miles and at 60 degrees latitude it is only 34.914 miles. 

30. Determination of Longitude.— Longitude is determined 
by finding the difference in time between the place in question 
and the meridian of Greenwich or some point whose longitude is 
known. Since the earth rotates on its axis once in 24 hours, in 
one hour a point on the surface must go l-24th of 360 degrees 
or 15 degrees, or one degree in four minutes. Hence the differ- 
ence in time expressed in hours multiplied by 15 will give the 
difference in longitude expressed in degrees. For example, a 
place two hours west of Greenwich is in 2x15 or 30 degrees west 
longitude. Longitude is commonly determined by a chronometer 
or by telegraph. Thus if one has a chronometer, which records 
Greenwich time, it is only necessary to determine carefully the 
time by this chronometer when the sun crosses the meridian at 
the point to be determined and multiply the difference between 
this time and 12 o'clock by 15 to have the longitude of the place. 
By the other method if a person in Buffalo should telegraph to 
St. Louis the exact time wh6n the sun is on the meridian at 
Buffalo and the person in St. Louis should substract this from 
the time when the sun is on his meridian and multiply the res- 
suit by 15, (if in hours, or divide by 4 if in minutes) he would 
have the difference in longitude between the two places. For 
accuracy an addition or subtraction must be made for the equa- 
tion of time (Sec. 33) obtained from the Nautical Almanac. 



THE EARTH AS A PLANET 29 

TIME 

31. The Julian Calendar.— The chronology of ancient 
history is very confusing and uncertain owing to the lack 
of any definite system for recording time. Julius Caesar, 
in the year 46 B. C, reformed the Roman Calendar into 
the Julian Calendar by making every fourth year contain 
366 days and the three intervening 365 days each. He 
also changed the beginning of the year from the first of 
March to the first of January and gave his own nam'C to 
the month of July while August was later named in honor 
of his successor, Augustus.* 

32. Gregorian Calendar.— The average length of the 
year in the Julian calendar was 365.25 days, which is 
about 11 minutes too long, a difiference which became 
manifest after several centuries. It was to correct this 
that Pope Gregory XIII, in 1582, made another change in 
which 10 days were dropped from the calendar, the day 
after March 11th being called March 2l3t. He modified 
the part in reference to the leap years so that the even 
centuries are leap years only when divisible by 400; thus 
the year 1900 according to the Julian calendar would be 
a leap year and have 366 days, but according to the Gre- 
gorian calendar it would have 365 days. The Gregorian 
calendar was at once adopted in the countries whose 
church adhered to Rome, but it was not adopted in the 
United States and England until 1752, and it has not yet 
been adopted in Russia and Greece. Hence in the his- 
tories we frequently find the letters 0. S., old style, refer- 
ring to the Julian calendar and N. S., new style, for the 
Gregorian calendar. 

* Augustus did not want his predecessor's month, July, to be longer than 
his own month, so he took a day from February and added it to August. 



30 PHYSICAL GEOGRAPHY 

33. The Day.— The sidereal day is the length of time 
it takes the earth to make a complete rotation with refer- 
ence to a star, that is, until the star is again on the same 
meridian. 

The solar day is the time of rotation with reference to 
the sun. Suppose the sun and a star on the meridian at 
the same time; during the interval until the star is again 
on the meridian, the sun will lack 3 min. 56.55 sec. of being 
there owing to the forward movement of the earth in its 
orbit. Hence the solar day is that much longer than the 
sidereal day. 

But solar days are not all the same length owing to the 
fact that the earth moves more rapidly in some portions of 
its orbit than in others. Since it is not possible to con- 
struct a clock that will follow all the variations of the sun 
from day to day, the length of our day is based not on the 
real sun but on a mean sun, moving through the heavens 
at the average rate of the true sun. This is called mean 
solar time, which is the time measured by our clocks. The 
difference between true solar time and mean solar time is 
known as the ^^ equation of time,'' and may be found in 
the almanac frequently marked '^sun fast" or ^'sun slow.'' 
It should be noted that even the mean solar day is actually 
determined by computation from the sidereal day. 

The civil day begins and ends at midnight rather than at noon 
as a matter of convenience. For the same reason the astronomical 
day begins at noon. It is also a matter of convenience to have a 
fixed place where the day changes or one day leaves off and an- 
other begins. 

The conventional day. It is always apparent noon on the 
meridian under the sun in its apparent passage around the earth. 
In imagination if we should follow the sun from noon on Monday 
around the earth until we returned to the starting point after 
24 hours, it would have been noon all the time and the question 
arises, "Where did we pass from Monday to Tuesday?" This 
place for the change of date was at one time fixed at the 180th 



THE EARTH AS A PLANET 31 

meridian east or west from Greenwich, that is, on the opposite 
side of the earth from the prime meridian, so that vessels crossing 
this line would add or subtract a day, depending upon which 
way they were going. It was found that the 180th meridian ex- 
tended through groups of islands belonging to the same nation, 
so that it was found advisable to shift it enough to have it come 
between nations and yet vary as little as possible from its first 
position. It is now called the international or intercalary date line 
and is shown on fig. 18. The day which changes here is known 
as the conventional day. 

The lunar day is the interval between successive passages of 
the moon across the meridian and is nearly an hour longer than 
the solar day. 

In both the sidereal day and the astronomical day, the hours 
are numbered from 1 to 24, thus avoiding the repetition of A. M. 
and P. M. This method of numbering the hours is used on the 
railways in Canada and Spain. Why is it not used in the United 
States? 

34. Standard Time.— If every point on the earth 
kept its time by the sun accurately, it would lead to a 
constant change of time as one travelled east or west. 
This was found to be so confusing on the railroads that 
some years ago a standard time was adopted, in which in- 
stead of changing the time every minute or second, or at 
every town, it is changed only once an hour and on the 
even hours from Greenwich. Thus in the eastern United 
States the time is based on that of the 75th meridian or 
the one passing through Philadelphia. Further west it is 
based on the 90th or St. Louis meridian, the 105th or Den- 
ver meridian, and the 120th or the one passing on the 
boundary between California and Nevada. The time, 
however, does not change on these meridians, as by so 
doing it would give the place immediately west of the line 
time nearly an hour different from sun time; so, to make 
the difference from sun time as little as possible, it is 
aimed to make the change of time midway between these 



32 



PHYSICAL GEOGRAPHY 



standard meridians, but on the railways it is most con- 
venient to make the change at the end of a division which 
is generally marked by some large or important city. (See 
fig- 17). 

The different names given to these time belts from 
east to west are Eastern, Central, Mountain and Pacific 
time and they are respectively 5, 6, 7, and 8 hours slower 
than Greenwich time. The accurate standard time is sent 
regularly at twelve o'clock each day from the naval ob- 




FlG. 17. Standard time belts in the United States. These 
belts continue at intervals of 15° longitude, making 24 in 
the circumference of the earth. Since they count both 
ways from Greenwich, they meet on the International Date 
line. (Fig. 18.) 

servatory at Washington to all the important telegraph 
offices in the United States. Standard time is used on the 
railways in most of the European countries. 

35. Magnetism.— Magnets are bodies which have the 
power of attracting iron and being in turn attracted by 
iron and in a much less degree the metals manganese, co- 
balt and nickel. Magnets are natural and artificial. The 
natural magnet is the mineral lodestone, or magnetite. A 



THE EARTH AS A PLANET 33 

piece of hardened steel may be made into a permanent mag- 
net by rubbing it on a piece of lodestone or better, by plac- 
ing it inside of a coil and subjecting it to a strong electric 
current. If the piece of magnetized steel or piece of lode- 
stone be now freely suspended on a pivot, it will form a 
magnetic needle, which, properly mounted, forms the 
mariners' compass. In the absence of a pivot it may be 
floated on a cork in a vessel of water. 

36. The Earth a Magnet.— If two compass needles are 
brought near each other, it will be seen that the north end of one 
repels the north end of the other, but attracts the south end. 
From this and other observations it is thought that the earth it- 
self is a great magnet and tnat the poles of the great earth mag- 
net are near, but not at the north and south poles of the earth. 
The north magnetic pole lies in the region north of Hudson Bay 
and west of Baffin Bay, Recent studies seem to show that it is 
not a fixed point but an area of considerable size. The north end 
of the compass needle points toward this magnetic pol-9, not the 
true north pole; hence in northern Greenland, the compass 
needle points south of west instead of north. In only a few 
places does the magnetic needle point to the true north and 
these points are connected by lines known as agonic lines. At 
all other points the needle varies from a true north direction, 
which variation is known as the magnetic variation or declination. 
Points having the same declination are connected by a line 
called an isogonic line. Note the position of the agonic and iso- 
gonic lines on fig. 18. 

Midway between the magnetic poles is the magnetic equator, 
where a needle suspended freely lies horizontal. As the needle is 
taken north from the magnetic equator the north end dips below 
the horizontal until a point directly over the magnetic pole Is 
reached, where it stands vertical. A needle so suspended as to 
swing freely in a vertical plane is called a dipping or dip needle^ 
and lines connecting points where the angle of dip is the same 
are isoclinal lines (see map on fig. 18). The isoclinals appear to 
be very nearly parallel with the isotherms, which would indi- 
cate some possible relation between the earth's magnetism and 
the heat on the surface. It is thought that in some way the ro- 
tation of the earth is a cause or the cause of its magnetism. 



34 



PHYSICAL GEOGRAPHY 




THE EARTH AS A PLANET 35 

MAPS AND MAP PROJECTION 

The representation of the different geographic features 
of the earth's surface on paper has been tried in a great 
many different ways in order to gain accuracy, combined 
with ease and rapidity of construction and economy of 
duplication. 

37. Globe. — The ordinary globe shows all features 
in their true horizontal relations better than any other 
method, but it is too expensive and too inconvenient for 
most purposes. A globe on which the mountains and 
plateaus are shown in relief and the ocean basins shown 
by depression is more realistic and likewise more expen- 
sive. 

38. Model.— Next to the globe, a model or relief map 
constructed of plaster, clay or papier-mache is one of the 
best ways of showing the surface features. A model may 
be made of the entire globe but more commonly it is used 
for small portions that can thus be shown on a larger scale. 
The advantage of the model is that it shows vertical as 
well as horizontal relations, but the objections are the ex- 
pense of construction and duplication and the inconvenience 
in carrying about or storing for reference. 

39. Maps. — Maps in which the features are shown 
on the surface of thin paper are the cheapest to make, 
much more convenient to handle and store away, and much 
less expensive than either globe or model. Hence thero 
are hundreds of times as many maps in use as models or 
globes. 

40. Projections.— Since the earth is spherical in form 
the attempt to represent its surface on a flat paper is at- 
tended with more or less distortion. A curved area spread 
out flat necessitates crumpling in some places and stretch- 
ing in others. To overcome this difficulty various methods 



36 PHYSICAL GEOGRAPHY 

have been devised for projecting the lines of the curved 
surface upon a flat one. Some of the various methods em- 
ployed are described in Appendix II. 

41. Scale of the Map.— The scale means the ratio of the 
distances between points on the map and the corresponding 
points on the earth. It may be given in units such as, 1 inch 
equals one mile or by fractions 1-63360 or 1:63360, which means 
that one inch on the map corresponds to one mile on the earth. 
The scale used in the construction of a map depends on the size 
of the area to be mapped, the purpose for which the map is 
wanted and the money available for its construction. An in- 
crease in the scale would mean an increase in the cost of con- 
struction. Many of the contour map sheets published by the 
United States Geological Survey are on a scale of 1:62500, but 
some have a scale of 1:125000 and some 1:250000 while others 
have a smaller scale. Maps of the whole United States have a 
much smaller scale, some being 1:2,500,000, some 1:7,000,000 and 
still others 1:14,000,000. The scale should always be marked on 
a map either by ratios or graduated lines or in both ways, ex- 
cept on small scale maps of large areas where the latitude and 
longitude lines indicate the scale. 

42. Contour Maps.— Elevations and depressions on 
the surface of the earth may be represented on the maps 
in several different ways. (1) By relief maps or models. 
On the flat surface relief may be shown by (2) shading, 
(3) hachures or broken lines and (4) contour lines. 

The model or relief map shows to the eye the features 
of relief better than any plan yet devised to show the same 
on a map. Of the different methods in use of representing 
relief on a flat surface, shown in fig. 19, the contour map 
is superior for many purposes. The hachures and the 
shading show hills and valleys but they do not show the 
height of the hills or the depth of the valleys. The con- 
tour map shows not only the relative but the actual ele- 
vation of any and every point on the area. 

The map is constructed by drawing lines connecting 



THE EARTH AS A PLANET 



37 




Fig. 19. Representation of an area (1) by shading, (2) by 
contour lines, and (3) by hachures. The method by con- 
tours is the only one that gives actual elevations. 



38 PHYSICAL GEOGRAPHY 

all points that have the same elevation above sea level. 
(Any known point may be taken as a base, but sea level 
is generally taken for convenience). The number of con- 
tour lines drawn on the map varies with the regularity of 
the slopes, the scale of the map, the heights of the hills, 
and the amount of detail desired in the map. The vertical 
distance between the lines known as the contour interval 
is sometimes 2, 5, or 10 feet on a large scale map of a 
small area with high hills. In a mountain district the in- 
terval is 50, 100, 500, or 1000 feet. (State the reasons 
why an interval of 5 feet is used on the Donaldsonville, 
La., topographic sheet, and 100 feet on the Charleston, 
W. Va., sheet). 

In the U. S. Geological Survey maps the contour lines 
are printed in brown to avoid confusion with streams, 
roads, and other lines. 

The first contour line (which is generally not in brown) 
on any continent or island area is the one marking the 
separation between the land and the sea, that is, the line 
marking the contour of the land. The second line would 
mark the contour of the land if the water should rise 10 
feet (or the space of the contour interval) and so on. If 
one considers the contour lines as marking the water level 
at successive stages, the significance of the name becomes 
apparent. A contour line never ends except at the mar- 
gin of the map. On a map of an island or of an entire 
continent all contour lines are continuous. 

43. The United States Topographic Atlas.— The United States 
Geological Survey contour map sheets in the topographic atlas of 
the United States will in time cover the entire area of the 
country on a scale of approximately one, two or four miles to the 
inch. This is one of the most useful maps published in this 
country and, because of its comparative accuracy, great detail, 
and small cost, its economic and scientific features should be 
known by all. It shows not only such topographic features as 



THE EARTH AS A PLANET 39 

rivers, lakes, roads, railroads, villages, often separate houses, 
ferries, bridges, mines, quarries, etc., but by contour lines the 
absolute and relative elevation of any and all points on the area. 

The atlas sheets are valuable aids in the study of geography 
and geology. Some of the points which the student can frequent- 
ly interpret from the contour map are: (1) The elevation of any 
and all points above sea level and hence of any point relative to 
any other. From one of these maps covering your home dis- 
trict it should be possible for you to tell how many feet your 
house is above or below the school house. Try it. (2) The 
steepness of the hillsides. (3) The location of the cliffs. (4) 
The extent of the drainage basins. (5) The topographic age or 
place in the cycle of erosion, whether young, mature or old. 
(6) The kind and structure of the rocks, whether igneous or 
stratified, whether folded, crumpled or not. (On some areas this 
cannot be determined from the map.) (7) Whether river piracy 
has taken place or is taking place. (8) Frequently the char- 
acter of the climate can be inferred along with the probable in- 
dustries carried on in the region, and the density of the popula- 
tion. 

They are serviceable and interesting to travellers. One 
learns in time to select the best roads from the study of the 
map, or in a region where roads are absent, to choose the best 
route for travel from place to place. 

The contour maps are of great service in laying out the 
routes for roadways, railways, electric lines, aqueducts, pipe lines 
and irrigating ditches. 

For these and other reasons contour maps should be thor- 
oughly studied so that the student can properly interpret them. 

REFERENCES 

1. Elements of Astronomy, F. R. Moulton, The Macmillan Co. 

2. Elements of Descriptive Astronomy, Howe, published by 
Silver, Burdett & Co. 

3. Manual of Astronomy, Young, Ginn & Co. 

4. New Astronomy, Todd, American Book Co., New York. 

5. Maps and Map Reading, Ravenstein, International Geog- 
raphy, D. Appleton & Co. 

6. Text Book on Geology, Chamberlin and Salisbury, Henry 
Holt & Co., Vol. 2, Chap. 1. 



CHAPTER II 
GROUNDWATER AND RIVERS 

44. Rainfall. — Where do the raindrops come from 
and where do they go? A vessel of water exposed to the 
air on a dry day is soon emptied. The water evaporates, 
that is, it passes into the air as invisible vapor. Evapora- 
tion is taking place over all the oceans, lakes, rivers, and 
moist land areas, sometimes rapidly, sometimes slowly. 
This is the source of supply for the rain. Lowering the 
temperature and pressure of the air causes the invisible 
moisture to form clouds and thus become visible; on 
further decrease of temperature and pressure, the water 
condenses and falls to the earth as rain or snow. Part of 
the precipitation is again evaporated from the surface and 
goes back into the air to be again precipitated elsewhere; 
part of it flows off on the surface directly into the streams 
and thence back to the ocean; part of it sinks into the 
earth and becomes groundwater. Some that falls as snow 
in cold climates forms streams of ice called glaciers that 
move slowly downward towards the sea, in some places 
even flowing into the sea. (See Chap. IV). 

The proportion of the total rainfall that is evaporated direct- 
ly back into the air is variable, depending upon such factors as 
the temperature of the atmosphere, the rate of the rainfall, the 
nature and condition of the surface, and especially the humidity 
or degree of saturation of the atmosphere. In a slow, drizzling 
rain a much larger portion will sink into the earth than in a 
dashing rain, which on a steep slope runs directly into the 
stream channels and on a flat surface stands in pools until it is 
partly evaporated into the air. 

40 



GROUNDWATER AND RIVERS 41 

Much of the rain that falls on sand or a broken rock surface 
sinks into the earth through the pores, while a large part of that 
which falls on a hard rock or baked clay surface runs off into 
streams or stands in pools on the surface. 

45. Groundwater.— The portion of the rainfall that 
sinks into the earth is called groundwater or underground 
water. It penetrates all rocks to a great depth, passing 
through the cracks, crevices and the pores of the rock. It 
moves down by force of gravity, capillarity, and pressure. 

A part of the groundwater finds its way to the surface 
again, some of it quickly, some of it after a long period of 
time and some of it remains below the surface indefinitely. 

Depth of groundwater. The lower limit of the ground- 
water occurs at depths of five or six miles below the sur- 
face, where the pressure from the overlying material is so 
great that even the finest pores are closed and the rock 
becomes too dense for the water to find its way through it. 

The water tahle. The successive rainfalls through the 
ages past have filled the rocks with water from the lowest 
limit up to a place where there is a balance between the 
annual supply from the rains and the loss through escape 
to the surface. This upper limit of the zone of saturated 
rocks is known as the water table, the level of permanent 
groundwater qr the permanent water plane. It serves to 
mark the upper boundary of the water zone or the zone of 
permanent groundwater, or the zone of saturation. 

In some places the water table is at the surface, in some 
places a few inches or a few feet below the surface, while 
in others it is several hundred feet below the surface. 

The depth of the water table depends upon the surface 
features, the climate and the rocks. It lies nearer the 
surface in the valleys and plains than it does in the 
mountains and plateaus. One reason why the former are 
more productive than the latter is that in the one in many 



42 PHYSICAL GEOGRAPHY 

places the water table is near enough to the surface to fur- 
nish moisture to the plant roots in dry seasons, whereas in 
the other during a prolonged rainless season the water 
near the surface is evaporated and the water table sinking 
too deep to be touched by the roots, the vegetation withers 
and dies. 

The level below which the moving water does not sink in dry 
weather is the permanent water table. In wet seasons this level 
rises towards, sometimes to the surface, to sink again during 
the dry season. This fluctuating upper surface of the water 
zone that varies with the seasons is known as the temporary 
water table. 

Aquifer. While the rocks below the water table are satur- 
ated, some of the layers are so fine grained and dense that the 
water can move through them only with extreme slowness, while 
other layers are coarse grained and permit free movement of 
the water. The latter are called aquifers (aqua, water; fero, to 
bear) and are very important economically since they are the 
beds which supply the wells and springs. (See figs. 32 to 35). 

46. Destructive Action of Groundwater.— While per- 
colating through the rocks the groundwater has in some 
places a destructive, in some places a constructive effect on 
the rocks through which it is passing. It comes in con- 
tact with some minerals that are soluble and takes them in 
solution, and this solution acts as a solvent for others. The 
material dissolved is carried away by the streams to the 
ocean, leaving cavities in the rock from which the material 
was carried. 

47. Caves. — Carbon dioxide, derived partly from the 
air and partly from the soil, dissolves limestone when it 
is brought in contact with this rock by the percolating 
groundwater which likewise acts as the agent to carry 
away the material after it is dissolved. In this way such 
caves as Mammoth Cave in Kentucky, Luray in Virginia, 
Wyandotte and Marengo in Indiana and Howe's Cave in 



GROUNDWATER AND RIVERS 43 

New York are formed. (See figs. 20, 21, 27a, 28 and 29.) 
The above are some of the best known caves in the 
United States, but there are hundreds of similar ones not 
so widely known. Nearly every bed of limestone has num- 
erous caverns large or small. In the upland areas of cen- 
tral Kentucky it is estimated that there are not less than 




I ,1 l \ 1 . ^ " L f L / ^_ 



TlG. 20. Vertical section of limestone showing caves and sink 
holes. Notice the vertical cliff and talus slope at the left. 

10,000 miles of limestone caverns. Limestone caves vary 
in length from a few feet to many miles, in depth below 
the surface from a few feet to several hundred feet. 

48. Life in Caves. — As might well be imagined caves 
frequently form retreats or hiding places for different 
animals, the most common being the bats, which fly abroad 
in summer nights but spend the days and the winter season 
in the caves where they sometimes cluster in great masses 
hanging frofti the roof of the cavern. Beetles, lizards, 
mice, wolves, bears, and foxes are some of the other animals 
that find a home in the caves. Blind fish are sometimes 
found in them. 

Savage man in ages past found shelter there. Relics of his 
handiwork, his implements and his carving on the walls, have 
been found in different caves often associated with bones, and 
pictures made by him of now extinct animals and even the bones 
of primeval man himself have been found. 

Corradingf Action of Cave Streams.— Besides the dis- 
solving action of the groundwaters on the rocks there is in 



44 



PHYSICAL GEOGRAPHY 




Fia. 21 Map of portion of the explored galleries in Mammoth Cave, Ken- 
tucky. Some of the galkries are much higher than others. There 
is a descent of many feet at the entrance, and one crosses several 
lofty hills in traversing the different chambers. The shaded portions 
contain water. This cave has been explored through nearly 200 miles 
of galleries. Scale about 1 V4 miles to one inch. 



GROUNDWATER AND RIVERS 



45 



places a corrading or wearing action on the bottoms and 
sides of caves similar to the work of surface streams. In 
some places several caves occur one above the other. The 
upper ones are dry while the lowest one frequently has 
a stream in part of the cave. Most of the corrading 
work done in the cave is not done by the permanent stream 
but by the temporary streams that pour in through open- 
ings in the roof during the rainy seasons. 




Fig. 22. "The Bottomless Pit." A limestone sink near Flagstaff, 
Arizona. The sink is 100 feet in diameter. The stream that 
disappears here is not known to reappear at the surface. In a 
wet season the water fills the pit, overflows and forms a lake. 
The flat in the picture is the silt-covered lake bed as it appears 
during low water. (Hackett.) 

Lost River in Indiana flows through limestone caverns for 
about 10 miles of its course, but in flood season when there is 
more water than can find escape through the underground chan- 
nel the surplus flows in the surface channel until the flood sub- 
sides when it disappears into the cave. Presumably there is 
considerable corrasion in such a cave. 

49. Sink Holes. — In nearly every limestone region 
where there are caves there are numerous basin-like or 



46 PHYSICAL GEOGRAPHY 

funnel-shaped depressions, called sink holes or swallow- 
holes. These are often shaped like a funnel, the large 
opening serving to catch the rainfall and to lead it into the 
narrow opening at the bottom, corresponding to the stem 
of the funnel. Through the sink hole the surface water 
drains into larger caverns. (See figs. 20 and 22.) 

A limestone surface much diversified by the action of 
the groundwater dissolving the rock along cracks and joint- 
planes, thus leaving many deep irregular fissures, is called 




Fig. 23. Surface of limestone outcrop at Syracuse Caves. The 
openings were formed by the ground- water dissolving the 
rock along the cracks and joint places. Some of the 
cavities extend to a depth of more than 100 feet. 

by the Germans the Karsten. There is no English word 
for this phenorTDcnon although it occurs in many places in 
New York and elsewhere in the United States. (See fig. 23.) 
50. Natural Bridges.— Natural bridges are formed 
sometimes by the breaking down of part of the roof over 
one of these subterranean streams. The portion of the 
roof that remains spanning the now open chasm is called 



GROUNDWATER AND RIVERS 47 

a natural bridge. Natural bridges are sometimes formed 
in other ways. (See figs. 24, 25, 146 and 148.) 

51. Constructive Action of Groundwater.— Some of 
the mineral material taken in solution by the groundwater 




Fia. 24. Natural Bridge, Va. The remnant of the roof of a 
cave formed under a waterfall. (U. S. Geol. Survey.) 

is carried in solution to the ocean while part of it is de- 
posited again, sometimes on the surface where the ground- 
water emerges, and sometimes underneath the surface. In 



48 



PHYSICAL GEOGRAPHY 




Fig. 25. The Caroline Bridge, Utah. The longest natural bridge in the 
world. Length 350 feet, width 60 feet, thickness 60 feet. Copy of 
painting made from photographs, sketches, and the measurements. 
The rock is light colored sandstone. (Courtesy of E. F. Holmes.) 




Fia. 26. Small calcite veins in limestone, formed by ground- 
water carrying carbonate of lime in solution and depositing 
it in cracks in the limestone. (U. S. Geol. Survey.) 



V^ OP THE 

UNIVERSITY 



OF 



CALIFOBJi^ 



ROUND WATER AND RIVERS 



49 



percolating through the rocks, the water and the carbonic 
acid gas in the water are under pressure when they take 
up more carbonate of lime than they can hold in solution 
under less pressure ; hence, when the water reaches a large 
cavity or the surface, where the pressure is lowered, some 
of the acid gas escapes into the air, some of the water is 
evaporated and part of the mineral matter is deposited. 
52. Veins.— When the mineral matter carried in solution is 
deposited in cracks or fissures in tlie rocks, it forms veins, in 




Fig. 27. Gold-silver vein near Ouray, Colorado. The vein 
is composed of quartz containing gold and silver. It 
is about 20 feet wide and thousands of feet in depth. 
It fills a deep fissure in the dark colored volcanic 
rock which shows on each side of the white quartz. 
Such veins are called fissure veins. 



which are formed compounds or ores of different metals such as 
gold, silver, lead, zinc, copper, etc. Mingled with the ores are 
variable quantities of other minerals known as gangue or vein- 
stuf consisting of calcite, fluorite, barite, quartz, and other min- 
erals, all of which are carried by the groundwaters into fissures 
and there deposited to form the vein. Man is largely dependent 
4 



50 



PHYSICAL GEOGRAPHY 



upon these veins for the supply of metals needed in the different 
industries, because in the original condition of the rocks the 
metals are so scattered and diffused that they cannot be profit- 
ably extracted until they are segregated as ores in the veins by 
the action of the groundwater. 




Fig 27a. Stalactites, stalagmites and columns in Marengo 
Cave, Ind. (Hessler and Smith.) 



53. Cave Deposits.— The water that very slowly drips 
from the roof of a limestone cave is partly evaporated and 
at the same time permits the escape of part of the carbon 
dioxide, which causes part of the lime carbonate to be pre- 
cipitated in the form of an icicle-like deposit called a 
stalactite. A corresponding projection built up on the 
floor of the cave is called a stalagmite. How can you 
prove that these are carbonate of lime? Many of the 



GROUNDWATER AND RIVERS 



51 



stalactites have a small hole running lengthwise through 
the middle. How do you account for it? 

If the stalactite and the stalagmite grow together 
forming a continous deposit from the roof to the floor it 
is called a column or 
pillar. A growth 
along the wall of the 
cave extending from 
the floor to the roof 
is called a pilaster. 
In some places this 
deposition goes on 
until the cave that 
was originally formed 
by the groundwater 
is completely filled by 
it. The more mas- 
sive and compact de- 
posits formed in the 
cave are quarried and 
used as onyx marble 
or Mexican onyx. (See 
figs. 27a, 28 and 29.) 

54. Spring Deposits.— The calcite or carbonate of 
lime is frequently deposited around springs, which are 
streams of groundwater appearing at the surface. The 
deposit of the spring is formed similarly to that in the 
cave, namely, by the escape of the carbonic acid gas and 
evaporation of some of the water, causing part of the 
dissolved lime to be deposited. It is frequently deposited 
on the surface of moss, leaves, or twigs because a large 
area is there exposed to evaporation. Such porous de- 
posits are called calcareous tufa. The more massive de- 
posits formed by springs and streams are called traver- 




Fig. 28. "Tower of Babei/ Marengo Cave, 
Indiana. Small stalactites on the roof. 
Stalagmites on the floor. A column ex- 
tending from roof to floor. (W. S. 
Blatchley.) 



52 



PHYSICAL GEOGRAPHY 



tine, a name which is sometimes used for all the deposits 
of carbonate of lime from solution. The Coliseum and 
St. Peters and many other large buildings at Rome are 
constructed of travertine, quarried from an extensive de- 
posit formed hy the springs at Bagni near Rome. ( See figs. 
30 and 177). 

Other materials than lime may be brought to the surface by 
the springs and deposited, such as iron oxide, sulphur, and silica. 
The silica-depositing springs are generally hot springs. 

55. Induration.— 

Mineral matter, such 
as silica, and the car- 
bonates of lime and 
iron carried in solu- 
tion by the ground- 
waters, is sometimes 
deposited in the open 
spaces between the 
grains in a bed of 
sand or gravel, ce- 
menting the particles 
together and thus 
changing it into a 
bed of sandstone or 
conglomerate. This is 
one of the principal 
ways in which beds 
of sediment are in- 
durated or changed to solid rock. Frequently the water 
is brought to the surface by capillarity, where it evaporates, 
precipitating the mineral matter in the pores. The fact 
that many sandstones are harder on the surface of the out- 
crop than in the interior of the bed is accounted for in 
this way. 




Pig. 29. "Pillar of the Constitution" in 
Wyandotte Cave, Indiana. A huge 
column of calcite, surrounded by small 
stalactites on the roof. (W. S. 
Blatchley.) 



GROUNDWATER AND RIVERS 



53 



In many places in the northern United States, portions of the 
glacial sand and gravel deposits are cemented by calcite de- 
posited from solution in the groundwater. The student may 
readily test this by placing a piece of the material in some dilute 
acid and noting the rapid effervescence, followed by the crumbling 
of the piece into separate grains or pebbles. It is this small per 
cent of lime that makes the glacial gravels better road-making 
material than the gravels from the creek beds. 




'Fig. 30. Travertine, carbonate of lime deposited by hot springs, 
Yellowstone National Park. (Detroit Pub. Co.) 



56. Reappearance of Groundwater at the Surface.-— 

What becomes of all the groundwater that sinks into the 
earth? Part of it is brought up by capillary attraction 
and evaporated from the surface in dry weather. Part 
of it is brought up through the roots and stems of plants 
and evaporated from the leaves. Part of it reaches the 
surface through artificial openings such as wells, artesian 
wells, mines, tunnels, and borings. Part of it combines 
chemically with minerals beneath the surface and is, tem- 
porarily, at least, locked up as water of crystallization. 
The water of crystallization is abundant in much of the 
loose surface rock, in clay and brown iron ore. It 
may be detected by taking a handful of clay soil, drying 



54 



PHYSICAL GEOGRAPHY 



it thoroughly and putting part of it in a test tube. Heat 
it over a gas lamp, when the water of crystallization will 
be separated and condensed as drops on the side of the 
tube. The amount of water may be determined by weigh- 
ing the sample before heating and after heating. The 
experiment is better performed with gypsum or limonite. 
Part of the groundwater penetrates the rocks to great depths 
and may not get back to the surface for many centuries; pos- 
sibly a small part of it may never return. A considerable and very 
important part of the groundwater is returned to the surface 
through springs and seepage, including hot springs and geysers. 

57. Wells are openings dug or bored from the sur- 
face down to a short distance below the groundwater table 
for the purpose of obtaining water; in the ordinary well 
the water stands as high and no higher than the water 
table. In fact, the best way to locate the water table in any 




Fig. 81. Variation of the water table with the seasons. 

a, temporary water table, April. Water in all the wells. 

b, temporary water table, June. W' is dry. c, permanent 
water table, September. W' and W are dry. v', Young 
valley with temporary stream in wet season only. Vo Larger 
valley contains stream until the temporary water table 
sinks below b. Va Mature valley, contains a permanent 
stream. 

region is by the level of the water in the wells when they 
are first opened. Sometimes excessive use of the water 
from a few large wells or many small ones may cause a 
lowering of the water table that is often of serious im- 
portance. 

Every well opening sunk below the water table will 
not prove productive because in some places the rocks 



GROUNDWATER AND RIVERS 



55 



are so dense that very little water can find its way through 
into the well. In porous rocks the wells are fed by water 
which seeps through the pores into the well opening and 
any well sunk into an aquifer or porous layer below the 
level of the water table will be productive. In the denser 
rocks the wells are fed by tiny underground streams. The 
well that strikes one or more of the little streamlets may 
have a bountiful supply of water while another close by 
that has missed the streamlets (or so-called veins of water) 
may be barren. 

The reason that some wells go dry in times of drought 
is that the water table sinks below the bottom of the well. 
Sometimes the reverse is true when the water table rises 
near or even to the surface and the well is filled to over- 
flowing. (See fig. 31.) 

58. The artesian well differs from the common well 
in that it occurs only in inclined strata down the slope of 




Fig. 32. Artesian wells W, W, W, in which the water supply 
comes from the different aquifers A, A', A". 

which the groundwater moves and enters the well under 
pressure which causes it to rise in the well above the 

water plane. The water 
may even flow out of the 
mouth of the well which it 
does frequently with con- 
siderable force. 
The name arte- 
sian is derived 

Fig. 33. A more favorable condition for artesian fpQjjj Artois 3 
wells than that shown in B, because the strata . ' . 

are not so highly inclined. prOVlUCC in 




56 PHYSICAL GEOGRAPHY 

France where the first well of this kind was bored. It was 
a very strong flowing well and for a long time only flowing 
ones were called artesian, but now the name is used for all 
deep wells where the water enters under pressure and rises 
considerably above the point of entrance and above the 
water plane. (See figs. 32 and 33.) 

The necessary conditions for an artesian well are (1) 
a layer of porous rock, the aquifer, through which the 
water can percolate freely; (2) the strata inclined to the 
horizontal; (3) the porous layer outcropping in a region 
of considerable rainfall, and (4) the aquifer overlain by 
a layer of rock less pervious; (5) all the strata dipping 
into the groundwater zone. It is immaterial what kind 
of rock is below the aquifer; if impervious it will hold 
the water above it ; if porous it will fill with water and in 
that way become impervious. (6) There should be no 
natural escape for the water between the outcrop and 
the well; (7) nor any obstruction to prevent the water 
reaching the well. (Study the diagrams, figs. 32, 34 and 
35.) 

The most favorable condition for an Artesian well is a gentle 
inclination of the strata as in fig. 34 and not highly inclined, as 



Fig. 34. Section from the Black Hills across portion of the 
Great Plains. The Dakota sandstone is the aquifer which 
receives the rainfall on its upturned edges in the Black 
Hills and carries it as groundwater hundreds of miles out 
under the dry plains where it is obtained by artesian 
borings. (After Darton). See Fig. 33. 

in Fig. 33 B because in the latter case the well must be near the 
outcrop in order to reach the aquifer or water-bearing layer, 



GROUNDWATER AND RIVERS 



57 



while in the first case the well may be many miles distant, even 
in a semi-arid or desert region, and yet get the water from the 
rain belt far away. Thus "there are artesian wells on the desert 
Of Sahara fed by the rainfall on the bordering mountains. The 




Fig. 35. Flowing artesian well at Woonsocket, South Dakota. It 
throws a 3-iiich stream to the height of 97 feet. This well is 
about 200 miles from the Black Hills. (N. H. Darton, Nat. Geog. 
Mag. Aug. 1905.) 



58 PHYSICAL GEOGRAPHY 

rain that falls on the sharp crested foothills of the Rocky Moun- 
tains and the Black Hills is carried in a bed of sandstone out 
under the great Western Plains for many miles where it is ob- 
tained from artesian wells, in some places even in sufficient 
quantities for irrigation purposes. The gently inclined beds of 
clay and sand on the coastal plain of Long Island and New Jersey 
favor productive artesian wells, which might even be sunk out 
in the ocean and furnish a bountiful supply of fresh water. Draw 
a diagram to illustrate this. (See figs. 35 and 33.) 

59. Springs. — Much of the groundwater returns to 
the surface in the form of springs, which are streams of 
groundwater emerging at the surface, and varying in size 
from tiny trickles to great rivers. Silver Spring in 
Florida and Mammoth Spring in Arkansas are each large 
enough to float a small steamboat. 

Sometimes the groundwater descends only a few feet 
below the surface until it finds its way back to the sur- 
face through a spring. Sometimes it descends thousands 
of feet before it is returned. 

60. Temperature of Springs.— The temporary springs feA 
from water near the surface vary in temperature during the year, 
becoming warmer in summer and cooler in winter. The perma- 
nent springs, however, are fed by water from below the per- 
manent water table which is generally below the zone of variable 
temperature, and the water has a uniform temperature through- 
out the year, generally about 55 degrees F. (Compare the tem- 
perature of the water in some of the springs in your neighbor- 
hood in the summer and in the winter months. Why does the 
water seem colder in the summer and warmer in the winter?) 
Some springs in a rocky region have a temperature considerably 
below 55 degrees because part of the water comes from melting 
ice whicn accumulates in the talus slopes during the winter and 
melts slowly during the summer. Sometimes the ice forms in 
limestone caves. There are several of these caves in the lime- 
stone near Syracuse, N. Y. 

61. Hillside Springs.— The very large springs gen- 
erally occur in the bottom of the valley. Why? But a 



GROUNDWATER AND RIVERS 



59 



great many small and medium sized springs emerge at 
different elevations on the hillsides, frequently a number 
of them at the same level. These are known as hillside 




Fig. 36. Hillside Springs on an area of five square miles at 
Eureka Springs, Arkansas. The shaded portion repre- 
sents a porous chert rock which forms hills 200 to 400 
feet above the underlying limestone. The latter forms the 
base of the hills 20 to 150 feet above the bottom of the 
valleys. The springs emerge on a thin bed of shale which 
separates the two rocks. 

springs and are caused by the groundwater in its descent 
from the surface meeting a bed of clay, shale, or other 



60 PHYSICAL GEOGRAPHY 

dense rock and following along the top of this layer until 
it emerges on the surface. (Figs. 36 and 37.) 




Tig. 37. Vertical section through Eureka Spring, Ark. Com- 
pare with Fig. 36. The groundwater percolates through 
the upper layer B faster than it can penetrate the shale 
layer. It moves along the top of the shale until it emerges 
at the surface forming springs at s, s, s, s. 

Seepage. — Ground water generally collects into little 
streams on the top of the impervious layer and the emerg- 
ence of such a stream at the surface forms a spring. Some- 
times, however, the water flows in a sheet along the entire 
surface of the dense layer and then instead of flowing out 
in streams, it seeps or trickles out along the line of outcrop 
of the layer in sufficient quantities to keep the surface wet, 
frequently forming a swamp or bog on the hillside. This 
is called a seepage spring. 

Fissure Springs.— Fissure springs consist of those in 
which the water in its underground passage, enters a 
fissure or crack leading to the surface through which the 
water emerges under hydrostatic pressure, as in an arte- 
sian well. 

62. Mineral Springs. — All spring water contains some 
mineral matter in solution but certain ones known as min- 
eral springs are characterized by an excessive amount of 
some common mineral matter, as carbonate of lime, car- 
bonate of iron, or hydrogen sulphide, or by the presence 
of some rare compounds like those of lithium. The most 
common mineral springs are the lime, sulphur, iron 



GROUNDWATER AND RIVERS 61 

(chalybeate), magnesia, carbonic acid, potash, soda, lithia, 
silica springs. Some of the mineral springs are hot and 
others are cold. 

Some have a wide reputation for the curative proper- 
ties of the water, for the benefit of which people travel 
long distances. In some places the waters are bottled 
and shipped to distant points. What mineral springs can 
you name in New York State? Your own State? Make 
a list of the places and the kind of springs. The springs 
shown in fig. 36 are widely known mineral springs. 

63. Hot springs are those in which the water has a high 
temperature, sometimes at or near the boiling point. Such 
springs occur in the region of active or extinct volcanoes, where 
the rocks have not yet cooled from the former highly heated con- 
dition. The circulating groundwaters coming in contact with 
these heated rocks below the surface are warmed and emerge 
in the spring as hot water. 

In some places where hot springs are remote from any vol- 
canic rocks, they may be caused (1) by intrusive molten rocks 
which have not reached the surface; or (2) by heat produced 
by friction in the bending and fracturing of the rocks in the fold- 
ing of mountains; or (3) by chemical action going on in the 
rocks through which the water is passing. Hot springs are found 
in mountainous countries, in Arkansas, Virginia, South Dakota, 
and many of the Rocky Mountain and Pacific States. 

64. Geysers. — Geysers are boiling springs that erupt 
intermittently. The water is thrown out periodically, 
sometimes to a height of several hundred feet. The 
eruptions take place in some geysers at quite regular in- 
tervals, while in others the intervals are very irregular, 
sometimes several hours or several days. 

The water in all geysers contains alkali (soda or potash) 
in solution, which in turn dissolves silica in the deeper 
portions of the circulation. When the heated silica-bear- 
ing waters approach the surface the decrease in pressure 
and loss of temperature causes some of the silica to be 



62 



PHYSICAL GEOGRAPHY 



deposited along the sides of the opening, making a smooth 
but crooked and irregular tube through which the water 
finds its way to the surface. 

The eruption of the geyser is caused by the high temperature 
in the deeper portions of the tube which causes the water to be 
heated above the boiling point. A portion of it finally changes 
to steam, the expansion of which lifts the plug of colder water 
in the upper part of the tube, causing it to overflow and thus re- 
lease the pressure on the water towards the bottom. This re- 




FiG. :i8. Old Faithful geyser in Yellowstone National Park. The 
white mound is composed of silica deposited from the waters 
of the geyser. (U. G. Cornell.) 

lease of pressure permits a large volume of steam to form sud- 
denly, and forcibly expel all the water from the tube. The 
water partially cooled in the air runs back, fills up the tube and 
stands until the bottom is again heated above the critical point, 
that is, the point where the water will change to steam under the 
existing pressure, when another eruption takes place. The crit- 



GROUNDWATER AND RIVERS 63 

ical point where water changes to steam at sea level on the sur- 
face of the earth is 212 degrees F., but deep beneath the surface 
under the increased pressure it may reach a temperature of 250 
degrees or more before it forms steam. Another possible explan- 
ation for some of the geysers is that steam accumulates in an 
underground cavity until it has sufficient force to overcome the 
resistance of the column of water when it violently expels the 
water from the opening. 

The constant loss of heat from the eruptions of hot water 
causes a decrease in the activity of the geyser which in time be- 
comes a hot spring and finally a cold spring. In the Yellowstone 
Park there are about 3,000 openings, some of which are geysers 
but the majority of them are now springs, some hot and others 
cold. A decrease in the activity of some of the geysers has 
been noted in the past few decades. Even "Old Faithful" that 
formerly erupted regularly every hour is now becoming irregular 
with sometimes an interval of an hour and a half between erup- 
tions. The decrease in activity of some of the geysers is balanced 
in part at least by increased activity in others. 

At present there are four geyser localities known in the 
world: one in the Yellowstone National Park in Wyoming, one 
in Iceland, one in New Zealand, and one in South America near 
the headwaters of the Amazon. 

RIVERS 

Part of the rain that falls does not sink into the 
gronnd at all bnt runs directly into depressions and 
through these to the sea. It mingles on the way with the 
water from springs, seepage, and the melting snows, 
which all together form the brooks, creeks, and rivers 
which fill the lake basins, and, running into the sea, re- 
place the loss that results from evaporation. The moisture 
evaporated from the ocean is carried as vapor through the 
air, falls as rain or snow upon the land, and is carried 
back to the ocean through the rivers. In this great cir- 
culating system, the rivers are of special interest to the 
geographer, because they have more to do with modify- 
ing the surface of the earth, and in sculpturing the beau- 



64 PHYSICAL GEOGRAPHY 

tiful and varied features of the landscape than any of the 
other parts of the system. 

65. Origfin of the River Valley.— River valleys may 
begin and grow in one or both of two ways: (1) The rain 
that falls on the border of a new land mass forms gullies, 
some of which deepen and lengthen and widen until they 
form river valleys. (2) The rain that falls on a new 
land area runs into existing depressions until they are 
filled, each overflowing into the next lower one, and from 
the lowest into the sea. The depressions when first filled 
with water are lakes, which in time are filled up with 
sediment. The streams cut gorges and valleys between 
the lake basins and finally through the filled basins until 
there is a continuous channel for the river from the inner- 
most rainfall to the sea. Probably in all river systems 
there is a combination of these two methods. Since the 
first method is the simplest it will be described, and the 
reader can apply the same principles to the second. 

66. Gullies. — The rain on the hillside washes away 
the softer material first and forms a little depression or 
gully. The depression gathers more water and so con- 
tinues to wear faster than the land on either side. After 
the gully is once started, the frost and wind and the other 
weathering agents (what are the others?) assist the rain 
in loosening and moving away the material at the sides 
and at the head of the gully. Refer to sections 71, 212 and 
213 for description of weathering and disintegration of 
rocks. 

The material so loosened is carried by the rain, assisted 
by gravity, to the bottom of the gully where it is swept 
along first by the temporary stream, and later by the per- 
manent stream, and, as it grinds against the sides and 
bottom of the channel, it corrades or wears away frag- 
ments of the rock and thus lowers the bottom of the gully. 



GROUNDWATER AND RIVERS 



65 



^jS^ 



'£v;Tr*; 



Fig. 39. Gullies and valleys in youtli in soft material in an 
arid region. Compare with Fig. 64. Though the annual 
rainfall is light it is concentrated in heavy showers. Bates' 
Hole, Wyoming. (U. G. Cornell.) 




Fig. 40. Telephoto view of gullies in the Bad Lands, South 
Dakota. Gullies in a dry climate. Compare with Figs. 39 
and 42. 



66 



PHYSICAL GEOGRAPHY 



As this process goes on, the bottom of the gully is finally 
cut below the water table into the zone of perpetual 
groundwater and then, and not till then, does the valley 
gain a permanent stream. The erosion then goes on con- 




JFiG. 41, Toad Stool Park, Adelia, Nebraska. Copyright 1898 by E. H. 
Barbour. Not every gully becomes a river valley. Observe the 
great number of gullies in proportion to the stream valleys in the 
hills in the background, 

tinuously throughout the year, and is supported by the 
springs and seepage that emerge at the sides of the valley 
and keep up the water supply during the dry season. 

The head of the gully or valley may extend back into 
the land area until it reaches a permanent divide where 
the streams flow in another direction. As the valley 
lengthens, tributaries develop along the side similar to 
the original gully, and tributaries to these in turn until 
the area is covered with a great system consisting of the 
main stream with all its tributaries. 



GROUNDWATER AND RIVERS 67 




Pig. 42. Gullies in hard volcanic rock on the side of Mt. Potosi, Ouray Co., 
Col. Compare witli FiGS. 39 and 64. The rocks shown in this view are 
all hard volcanic lavas. 

It must not be inferred that every gully becomes a 
river valley. To one such there are thousands that are never 
anything but gullies, and possibly not large ones at that. 

67. Definition. — A river is a stream of water together 
with the rock waste which it carries. It has its source in 
the springs, seepage, rainfall, and snowfall in and around 
the upper end of its valley. Not infrequently a lake is 
the source of a river, but generally it is only part of a 
river, in a wider and expanded portion of the valley; and 
the stream or streams that flow into the lake are but the 
headwaters of the river that flows out of it. Some rivers 
have their sources in melting glaciers. Small streams are 
commonly called creeks, brooks or rivulets but the distinc- 
tion is seldom made in geography because it is merely one 



68 



PHYSICAL GEOGRAPHY 



of size and what is called a creek in one locality may be 
larger than one called a river elsewhere. 

The land over which the stream flows is the bed and that im- 
mediately bordering the stream confining it to the bed forms the 
baiiks. The mouth of the river is the place where it flows into 
the sea, a lake, or another river. A river basin is all the land 
drained by the river and its tributaries. A river system includes 
the main river and all its tributaries. A divide is the parting be- 
tween the surface waters of two valleys or basins. It may be a 
sharp ridge in mature topography, or a broad flat or plateau in 
young topography, or it may rarely be a body of water. The 
divide between the Amazon and Orinoco rivers is so low in one 
place that it is possible to cross it in a boat; in fact, it shifts 
back and forth over a considerable length of water depending on 
the local rainfall and the height of the water in the two rivers. 
(Trace out and describe the divides on the Charleston and Ottawa 
or Fargo sheets of the Topographic Atlas. The first is an ex- 
ample of mature topography and the other two of youthful topog- 
raphy. See Sec. 85) 











































LAK 


ETEA 


R OF! 


HE CI 


OUDS 






































































































-^ 
























^ 


/ 
























o 

z 


/ 










^ 
















i 






>> 






//i 


dsaa. 


5_ 


o 


^ 






-" Mohair/: 



Fia. 43. Profile of the Hudson-Mohawk River. Observe the 
contrast between the upper and lower Hudson and between 
the upper Hudson and Mohawk. Vertical scale 2000 feet 
to one inch. Horizontal scale 100 miles to one inch. — 
(U. S. Geol. Survey.) 



GROUNDWATER AND RIVERS 



69 



68. Profile of a River.— The slope of a river channel or the 
angle of the inclination to the horizontal is one of the chief 
factors in determining the velocity of a river; the steeper the 
slope the more rapid the current. A line representing the slope 
of a river channel from the headwaters to the mouth of a river 
is called the profile. In general the slope is steeper near the 
headwaters of the river and decreases towards the mouth. Com- 
pare the profiles of some of the principal rivers of the United 
States and draw the profile of a river from furnished data. (See 
Bulletin 44 of the Water Supply and Irrigation, Papers published 
by the U. S. Geological Survey.) 




Pig. 44. East branch of Limestone Creek, Manlius, N. Y., show- 
ing reach or pool in foreground and rapids in the back- 
ground. Very little, almost no erosion is going on at the 
reach. On the rapids the rocks are being ground to pieces 
and the finer portions carried to a lower level. 



69. Reaches and Rapids.— A river is rarely if ever 
of uniform slope from the mouth to the head, but gener- 
ally consists of a series of alternating pools or stretches of 
quiet water called reaches separated from each other by 
ripples or rapids where the current flows swiftly. Por- 
tions of the pools are generally being filled with sand and 
mud, and the rapids are cutting the channel deeper and 



70 



PHYSICAL GEOGRAPHY 




Fig. 45. West branch of Limestone Creek, 
Manlius, N. Y., showing rapids in the fore- 
ground. At this point the stream, owing 
to its curvature, is cutting sideways and 
widening its valley at the reach. Notice 
the undercutting of the bank on the right. 





■r 




MmB^ 




1 — i 



Fig. 46. Potomac River, Barnum, Md., showing rapids at low 
water and the tools which the stream uses in degrading its 
channel. Most of the work done by the stream is accom- 
plished during the high water stage. (Maryland Geological 
Survey. ) 



GROUNDWATER AND RIVERS 71 

slowly receding towards the headwaters. In the pools or 
flats the river is aggrading its channel, that is, depositing 
sediment and raising its bed; on the rapids the stream is 
degrading its channel, that is, cutting it deeper. Some 
portions of the river course are neither aggrading nor 
degrading but are graded. As a stream advances in age 
the graded portions tend to increase in length and when 
they are all connected and all rapids have disappeared, 
the stream is completely graded and has reached base 
level. (Study figs. 44, 45 and 46.) 




Fig. 47. Streams flowing into a lake form deltas 
and deposit sediment until the lake basin is 
filled to the level of the outlet. Compare with 
Fig. 48. 

70. Base Level. — The base level is reached by a river 
when the rapids disappear and as a stream of clear water 
it flows with a slow motion, neither eroding nor deposit- 
ing material, over a plain having a gentle, uniform incli- 
nation from the headwaters to the mouth. It is the low- 



72 



PHYSICAL GEOGRAPHY 



est level to which the river mechanically erodes the land 
over which it is flowing. (Some writers define the term 
hose level as the level of the sea or other body of water 
into which the river flows. For a discussion of the mean- 
ing of the term see Davis, Journal of Geology, Vol. x, p. 

77.) 

71. Work of Rivers.— The work of a river consists in dis- 
secting the upland areas and carrying the material along with 
the excess rainfall to the lowlands and finally to the sea. In this 




Fig. 48. After the lake is filled, the stream from 
the outlet is supplied with sediment, degrades 
its channel, and then picks up and carries 
onward the sediment formerly deposited in the 
lake basin. Compare with FlG. 47. 



work it is assisted by the weathering agents, such as wind, frost, 
heat of the sun, gravity, chemical action, and animal and plant 
life, which cause the rocks to disintegrate and crumble into 
fragments. The rain washes the loose materials into the stream 
channels, and the current carries them toward the sea. The 
journey is not a continuous one, because the material is dropped 
and picked up again many times between the mountain and the 



GROUNDWATER AND RIVERS 73 

sea. Thus, when a river flows into a pond or lake, nearly all the 
sediment is deposited until the lake is completely filled, after 
which the river, under new conditions, picks up the materials and 
carries them on to a lower level. Most of the transportation is 
done during flood season, but the disintegration goes on contin- 
uously throughout the year. (See figs. 47 and 48.) 

72. Corrasion.— The material carried by the stream, 
the sand, mud, and gravel, corrades or grinds away the 
rocks in the channel. The sand and pebbles are the tools 




Fig. 49. Pot hole formed in rock by the grinding action of the 
pebbles and sand in the eddying waters on the rapids. (U. S. 
Geol. Survey. ) 

that do the grinding, while the water acts as a carrier. 
In the rapids, eddies are formed and pebbles are caught 
and swirled around in the w^aters until a circular depres- 
sion or pot-hole is formed which may be from a few inches 
to many feet in depth and diameter. Pot-holes are also 
formed underneath glaciers where a surface stream de- 
scends through a crevasse or moulin. (See Sec. 126, fig. 
116). Deep, narrow gorges like Ausable Chasm and Wat- 



74 PHYSICAL GEOGRAPHY 

kins Glen indicate rapid corrasion or down-cutting by the 
stream. 

73. Transportation.— Rivers transport materials in 
several different ways: (1) Floating on the surface. 
Great quantities of vegetable material are carried in this 
way and at times limited quantities of earthy material. 
Where the water rises gradually over a dry sand deposit, 
little cakes and patches of the dry sand are floated away 
on top often a long distance before they are disturbed 
and sink. Place a sewing needle gently on the surface of 
a cup of water and it will float in the same way. When 
the river undercuts its banks and a part of the forest or 
vegetable covering slides into it, the earthy material 
clinging to the roots is carried away with the floating 
trees and grass. In cold climates the blocks of ice borne 
away on the spring floods frequently carry fragments of 
rock and earth long distances down the stream before the 
ice melts and drops them in the channel. (2) Great 
quantities of mud and fine sand are carried along sus- 
pended in the water borne up by the many upward whirls 
in the current. (3) Boulders, pebbles, and sand are 
rolled and pushed along the bottom, and hence are being 
continually worn rounder, smoother and smaller. The 
larger boulders are moved in the more rapid portions of 
the stream as in the mountain torrents, while through 
the flood plain in the lower portion of the river, the 
burden is mostly sand and mud. It is surprising how 
great a quantity of sand is being moved along the bottom 
of the lower courses of a great river like the Mississippi 
or the Missouri. (4) Besides the above there is the in- 
visible load which the river carries in solution, consist- 
ing of compounds of lime, iron, magnesia, soda, potash 
and minute quantities of other materials dissolved from 
the rocks. This part of the work goes on all the year. 



GROUNDWATER AND RIVERS 75 

even during the low water stage in the dry season when 
the waters are free from sediment. 

74. Transporting Power of Streams.— The carrying 
power of streams varies greatly with the velocity of the 
water, the velocity being frequently a factor of the vol- 
ume. In the Johnstown, Pa., flood the great volume of 
water from the broken reservoir moved down the same 
slope as that over which the Conemaugh river flows at all 
times. Yet the force of the flood, due to the increased 
volume of water, was sufficient to twist railroad irons, move 
freight cars, and even railway locomotives, and cause enor- 
mous destruction of property. The increase in the carry- 
ing power is in a much greater ratio than the increase in 
velocity, hence the great destruction wrought by streams in 
flood time. The carrying power increases as the sixth 
power of the velocity. That is, if the velocity is increased 
10 times the carrying power is raised 1,000,000 times. 

The transporting power of streams having different velocities 
is shown by the following: 

3 inches per second will just move fine clay. 
6 inches per second will move fine sand. 

12 inches per second will move fine gravel. 

24 inches per second will move pebbles an inch in diameter. 

36 inches per second will move pebbles as large as an egg. 

10 miles per hour will move masses of 1^/^ tons. 

20 miles per hour will move masses of 100 tons. 

The load carried by a stream is often a small fraction of the 
carrying capacity of the stream. If the water is flowing over 
hard rock it may be unable to pick up any load. It is not always 
the rapid current that is carrying the greatest load; although it 
has the capacity it may not have the load to carry. 

75. How the Energy of the Stream is Expended.— 

Part of the potential energy of the stream is used up in 
transporting its load, and part in corrading its channel. 
If the stream is full loaded, that is, has all the material it 



76 



PHYSICAL GEOGRAPHY 




Fig. 50. An underloaded stream which is corrading its channel 
in granite. View in the canyon of the North Platte River. 
(U. G. Cornell.) 




Fia. 51. An overloaded stream, Tncninpai^'hre Creek, Ouray, 
Col. Above this point the stream flows through narrow 
canyons, similar to that in Fia. 50, carrying much sediment 
which is deposited in this broader part of the valley below 
the canyons. Along with the bowlders, gravel and sand 
there is considerable driftwood deposited. 



GROUNDWATER AND RIVERS 



77 



can carry, it will not do any eroding. If it is overloaded 
it will deposit part of the load; if it is underloaded, the 
excess energy will be expended in deepening and widening 




Fig. 52. An overloaded stream, Uncompaghre Creek above Sneffels, Col. 
The same stream as in Fig. 51 but nine miles nearer the source. 
Note the narrower valley, the steeper slope in the channel and the 
coarser material deposited in the channel. The white spot near the 
middle of the picture is a cataract. 

the channel by corrasion. The Missouri river is over- 
loaded across the plains and underloaded on most of its 
course through the Rocky Mountains. Hence it is deep- 
ening its channel through the mountains and building it 
up across the plains. (Explain the different ways in which 
a stream may become overloaded. See figs. 51, 52 and 53). 
76. Deposits Made by the River.— Probably the lar- 
gest deposits made by the river are those made on the 
flood plain and in the delta, but before the sand and mud 



78 



PHYSICAL GEOGRAPHY 



reach either of these stopping places they may have been 
dropped and picked up many times and had many rough 
tumbles and a varied experience in the upper courses of 
the streams. 




Fig. 53. An overloaded stream, Taughannock Creek, N. Y. 
The mass of material in the channel has been swept by 
the stream in flood season from the narrow gorge in the 
hills in the background. (E. R. Smith.) 

The bulk of the material moved by a river is carried in flood 
time because of the increased volume and velocity of the water, 
but it is frequently moved only a short distance, to be dropped 
until moved again by a subsequent flood. Sometimes a stream 
may be so overloaded that it will deposit part of the load and 
pick it up again, when it has no load, even at low water. 

77. Flood Plains.— All streams as they approach old 
age develop more or less extensive plains bordering the 
channel,— plains so low that they are covered with water 
during flood season in the river, hence the name flood 
plain. The water covering the flood plain is the overflow 
from the channel and has lost much of its former velocity, 
because, first the water is shallower than in the channel, 
and, second, the vegetation acts as a check on the velocity. 



GROUNDWATER AND RIVERS 79 

Hence much of the sediment— the sand and mud— is de- 
posited, forming the rich alluvial soil that makes the flood 
plains such rich farming regions. 




Fig. 54. Flood plain of stream flowing into Chesapeake Bay, 
Calvert Co., Md. Rivers generally have a meandering 
course on a flood plain. (Maryland Geological Survey.) 

The flood plains are very large, covering many thou- 
sands of square miles on the lower courses of old and large 
rivers like the Mississippi and the Nile. In general the 
flood plain narrows in ascending the river. Frequently 
in the middle and upper courses of the streams there is a 
narrow flood plain on one side of the stream while the 
other flows against the bordering cliff. 

Why are there no large flood plains on the Hudson and the 
Niagara rivers? Do you know of any large flood plains on any of 
the rivers in New York? (See flgs. 54, 55, and 57 and map of 
the lower Mississippi River.) 

78. Meanders. — When a river has graded a portion 
of its course and formed a flood plain, it ceases to corrade 
the bottom of its channel at that place but may continue 
to cut at the sides or banks. Where the current is de- 
flected to one side it cuts away the bank at that point, pro- 



80 PHYSICAL GEOGRAPHY 

ducing a curve which deflects the current across to the 
opposite bank below, where another curve is formed. In 




Pig. 55. Meandering stream — Coal Creek on the Laramie plains, 
Wyoming. This stream is utilized to transport the logs from the 
forests in the adjoining mountains to the mills far out on the plains. 
U. G. Cornell.) 



Fig. 56. Meanders, M, M', M", R, R, R, former course of the 
river. C beginning of a cut-off. D is a cut-off. The ends 
of the lagoon LL are being filled with sand and mud at a, a. 

both places the bank is worn away and the stream in time 
becomes quite crooked or meandering. While the stream 
is cutting the outside of the meander curve, sand and 



GROUNDWATER AND RIVERS 81 

gravel are being deposited on the inside of the curve 
(why? study the diagrams) and the whole channel is 
gradually shifted into or even across its flood plain. The 




Fig. 57. Map of portion of the Mississippi River, showing 
meanders and cut-offs. The lakes are ox-bow lakes and 
portions of the abandoned channel. Note the sand de- 
posits a, a, a, on the inner bank at many of the curves. — 
(After the Miss. River Com.) 

meander curve once started continues to increase its cur- 
vature until the stream cuts across the neck between two 
approaching curves and thus straightens that portion of 
the channel. The cut-off is gradually silted up at the 
ends, forming first a lagoon and later an ox-bow lake. 
These ox-bow cut-offs, so common on the flood plains of 
6 



82 PHYSICAL GEOGRAPHY 

large rivers, may frequently be observed on small brooks 
where they are following a winding course through a 
meadow. Study the Mississippi River maps and figs. 55, 
56 and 57. 

When the meander curve reaches the outer limit of the flood- 
plain of the river, it begins to undercut the river bluff and thus 
widen the valley. Study meander curves on nearby streams; 
where possible observe them at intervals of a few months or 
years. Suggestion to the teacher — take photographs each year 
for comparison. (See fig. 45.) 

79. The natural levee is formed on the flood plain 
on the immediate bank of the stream. In flood season the 
river overflows its banks and spreads out in a thin sheet 
of water, moving slowly down the valley over the level 
area, but the water in the channel moves much more 
rapidly than that on the flood plain because it is much 
deeper. At the contact of the swiftly moving water of 
the channel and the slower moving water of the flood 
plain, there is a deposition of sand and mud due to the 
check in the velocity. The increase in size of this em- 
bankment is aided by the dense growth of willows, alders, 
and other bushes along the bank which catch the drift and 
add it to the bank as well as aid in checking the current 
and adding to the deposit of the sediment. 

80. Artificial Levees.— Man attempts to improve on nature's 
methods by adding material to the top of the natural levees, pro- 
ducing artificial ones in the endeavor to keep the river in its 
channel and prevent it from overflowing the flood plain. As the 
levee is built up, the river deposits material on the bottom of the 
channel, making it necessary to keep adding to the top of the 
levee until it is sometimes built up many feet above the border- 
ing area, so that steamboats in the river are sometimes above 
the level of the neighboring farms. A break in such a levee 
(called a crevasse) often proves to be very destructive to the 
bordering flood plain. Sometimes the river even leaves the exist- 
ing channel permanently and forms a new channel elsewhere to 



GROUNDWATER AND RIVERS 



83 



repeat the process, and in this way by a repetition of such changes 
finally raise the level of the entire plain. 



^ 


"■ --%- 


^l^^^^y^ 




^ 


[ 


"- ''"^*i 


it 


^^^^^^^ -*'- 




- -^^"^^^^^f,,,^ 


-^^^i^^^—r. - ';■ 






,.„-— ^-w-5| 





Fig. 57a. Strengthening a levee on the Mississippi River at Lagrange, Miss., 
during the flood season. (National Geog, Mag., Oct., 1907.) 

81. River Swamp.— The river swamp generally oc- 
cupies the outer borders of the flood plain next to the 
river bluffs, because the building up of the natural levee 
along the banks of the channel causes the plain to slope 
from the channel towards the bluff and hence causes a 



5 c; £ 



Fig. 5S. Cross section of a river valley — showing the flood 
plain, river swamp, natural levee (the small elevation 
on each side of the river channel), the position of the 
water table (WP). Liable to be springs or seepage and 
hillside bogs at S S. 

part of the rainfall on the flood plain to drain away from 
the stream instead of toward it. Likewise the water 



84 PHYSICAL GEOGRAPHY 

from the bluffs, unable to find its way across the natural 
levee, accumulates in the depressions along the base of 
the bluff and aids in forming the river swamp. 

82. Levee Lakes.— The upbuilding of the flood plain along 
the channel banks sometimes forms a dam across the mouth of 
a tributary, and thus produces a lake on the tributary, while in 
other places, instead of forming a lake the tributary flows for 
many miles parallel with the main stream between the levee and 
the bluffs until it finds a place where it can penetrate the levee 
bank into the main stream. The above points should be studied 
on detailed maps of flood plains such as the Donaldsonville and 
Point a la Hache, La., contour sheets of the topographic atlas 
and the sheets of the Mississippi River Survey, and the streams 
near the schoolhouse. (See fig. 221.) 

83. Deltas. — A river flowing into a body of still water 
as a lake or the ocean, w^here there are no strong tides, 
deposits all of its load of sediment, building up an accu- 
mulation called a delta. There is generally no sharp line 
of separation between the delta and the flood plain; the 
latter has been built up on the land and the former has 
been formed in the sea or lake. The river divides on the 
delta and finds its way into the sea by several, sometimes" 
by a great many channels called distributaries. The head 
of the delta is frequently located where the first distribu- 
tary leaves the main channel. 



Pig. 59. Cross section of a delta showing position 
of the beds. A, top-set beds. B, fore-set beds. 
C, bottom-set beds. S, sea level. 

Deltas, like flood plains, have a fertile soil and fre- 
quently support a dense population. Both are fertile 
because they are composed of the rich surface soil from 
other parts of the basin. It contains much humus 
and is frequently renewed. 



GROUNDWATER AND RIVERS 



85 



The structure of the delta as shown by the diagram 
is characteristic. The middle portion consists of mingled 
sand and mud beds formed at the end of the delta where 
the river current first meets the still water. 

At this point a larger part of the load is dropped than 
at any other and the material comes to rest in inclined 
layers, the fore-set beds, dipping towards the open water. 




Fig. 60. Walnut canyon, near Flagstaff, Ariz., showing delta 
structure in the sandstone of the canyon walls. Commonly 
known as cross bedding or false bedding. (A. E. Hackett.) 



But some of the fine materials is carried out into deeper 
water forming the mud layers that sometimes reach con- 
siderable thickness and great extent, the hottom-set beds, 
which form the submarine delta. 

The delta of the Indus River has built up the submarine por- 
tion nearly to the sea level over such an extensive area of now 
shallow sea that in places large vessels cannot even get within 
sight of the shore. 

Over the top of the inclined beds is a deposit of horizontal 
beds, the top-set beds, made by the river in flood time and not 
essentially different from the flood-plain deposits. All these 



S6 PHYSICAL GEOGRAPHY 

structural features may be observed in the mud deposit formed 
in the pool on the roadside by a summer shower. 

84. Alluvial Fan.— An alluvial fan is somewhat like 
a delta. It is formed at the point where a mountain tor- 
rent or stream with a steep slope carrying a great deal of 




Fig. 61. An alluvial fan at Ouray, Col. The fragmental ma- 
terial on which the trees are growing was swept out of the 
narrow canyon faintly visible near the right of the view. 

sediment flows out on a valley floor or flood plain of a 
larger stream, or on a plain of any kind. The velocity of 
the swift current is checked suddenly and the load nearly 
all deposited at the border of the plain, building up a 
fan-shaped mass over which the stream flows in shifting 
channels in wet weather. In dry weather the water fre- 
quently disappears from the surface entirely, flowing 
through the mass as groundwater. It differs from a delta 
in being built up on the land instead of in the water, hence 
the material is not well stratified, but consists of a jumbled 
mass of coarse and fine deposit almost devoid of any 
stratification. The delta of a large stream rarely con- 
tains material coarser than sand or very small pebbles 



GROUNDWATER AND RIVERS 



87 



but the alluvial fan sometimes contains boulders mingled 
with pebbles, sand and mud. If the stream forming the 
fan is small and descends a very steep slope the deposit 
will have a steep surface and resembles the section of a 




Fig. 62. Tahis conos in the Rocky Mts., Col. The rock 
fragments loosened by the frost and other weathering 
agencies roll down slight depressions on the moun- 
tain side and accumulate in conical mounds at the 
base. 

cone, when it is called an alluvial cone. Talus cones are 
similarly formed by gravity and rainwash at the base of 
steep mountain or hill slopes. (Compare figs. 61 and 62.) 

85. Life History of a River— The Cycle of Erosion.— 
The successive changes which a stream undergoes from 
the time it starts on an upland area until the upland has 
been reduced to a lowland constitutes the life history of 
the stream. It has a beginning, a period of development, 
decline and disappearance. It is customary to distinguish 
at least three different stages in the cycle as youth, matur- 
ity and old age. 

Youth is the period of rapid growth in the beginning 
of the cycle. Some of the characteristic features of this 



88 



PHYSICAL GEOGRAPHY 



stage are narrow V-shaped valleys, cataracts and rapids, 
lakes and swamps on the upland and inter-stream areas, 
few tributaries, and broad stretches of undrained or poor- 




FlG. 63, In the canyon of the North Platte River. Stream 
in youth in hard rock on an arid plateau area. (U. G. 
Cornell.) 

ly drained country bordering the valley. The conditions, 
of course, are different on a stream developing on a plain 
from the one on the plateau or mountain, yet the youthful 
stage of each can be recognized from its advanced or ma- 
ture stage by some or all of the above features. Fox 



GROUNDWATER AND RIVERS 



89 



.2lmt^.i^, mm- liii 






■^ 


" -r^ 



Fig. 64. Valley in youth in soft material formed by rain wash in a 
humid region, Calvert Co., Md. Compare with FiGS. 63, 39, 40, 
and 42. (Maryland Geological Survey.) 

River on the Ottawa, 111., topographic atlas sheet is a type 
of topographic youth. (See figs. 63 and 64.) 

Maturity of the streams is characterized by the ab- 
sence of lakes and swamps, which have been filled or 
drained, the absence of cataracts, decrease in the num- 
ber of rapids, increase in the number of tributaries, and 
complete dissection of the inter-stream areas. The di- 
vides are sharp ridges. There is some shifting of the 
divides. The sides of the valley are steep, with many 
cliffs and talus slopes; small flood plains have developed 
in places, the river is beginning to meander; the cross 
sections of the valley are changing from a V-shape to a 
U-shape. In the mature stage the erosion is at the maxi- 
mum, and there is the greatest percentage of steep hill- 
sides over the area. A large per cent of the rainfall is 
conducted rapidly into the stream channels resulting in 



90 PHYSICAL GEOGRAPHY 

destructive floods in the wet season. " The tributaries of 
the Ohio River in West Virginia are good examples. In 
the mature stage the upland plains have almost or entire- 
ly disappeared, that is, they have been dissected by the 
stream and its tributaries into hills and valleys, the tops 
of the hills being the remnants of the former upland 
plains or plateaus. 




Fig. 65. Pine Creek, Pa. A revived stream approaching 
maturity in the Alleghany plateau. 

In old age the narrow, sharp divides of the mature 
stage are cut down into low rounding hills with gentle 
slopes; the talus slopes extend to the top of the hills, the 
cliffs have disappeared; flood plains increase in size with 
corresponding increase in the meanders of the river, and 
formation of ox-bow lakes. Deltas increase in size and 
natural levees and river swamps become prominent. 
(Study the diagrams showing changes in profile and in 
cross section, also the contour maps cited, and the streams 
seen in your field trips). In extreme old age the hills and 



GROUNDWATER AND RIVERS 91 

uplands are nearly all worn down to the level of the val- 
leys, when the whole area is called a peneplain, (Sec. 216) 
the final stage of erosion being that of base level. (Sec. 70.) 
In old age the upland plains have disappeared and low- 
land plains have formed and are increasing in size. 
Lakes and swamps are forming on the flood plains, but 
have all disappeared from the upland. 



Fig. 66. Cross sections of a valley in 
youth AA, maturity B, and old age D. 
M is a monadnock. (See Section 216.) 

86. Accidents or Interruptions to the Cycle.— The 
cycle of erosion on a hard rock area is so long that there 
is generally an interruption of some sort before any river 
completes the whole cycle. The principal interruptions 
to the cycle are an elevation or depression of the whole 
or part of the area drained by the river. There is abun- 
dant evidence that many land areas have been elevated and 
depressed several times during their history. In a great 
many places at the present time, the land is slowly rising 
and in other places sinking. (Sec. 217.) 

The depression of the lower portion of a river basin 
carries part of it below the level of the sea which extends 
up the valley as a bay, such as Delaware or Chesapeake 
Bay, or an estuary such as the Hudson River below Troy. 
The river is dismembered of its tributaries on the drowned 
portion. Thus the Potomac, Rappahannock,. York, and 
James rivers that now flow into Chesapeake Bay were 
tributaries of the Susquehanna River before it was 
drowned. Study contour sheets for examples of dismem- 
bered rivers. 



92 PHYSICAL GEOGRAPHY 

If only the middle or upper portions of a river basin 
are depressed, or depressed more than the lower portion, 
the river will permanently overflow its flood plain, and 
swamps or lakes will be formed. 

87. Revived Rivers.— By the elevation of a river 
basin the stream and its tributaries are revived or rejuv- 
enated and enter upon a new cycle. The velocity of the 
current is quickened, and it begins to degrade and lower 
its channel as it did in the beginning. Where the river 
had reached old age and was meandering on its flood 
plain before the elevation it will cut its way down in the 
channel it occupied and intrench itself in the same wind- 
ing course that it had on the flood plain. See Canado- 
guinett Creek on the Harrisburg, Pa., topographic sheet. 

88. Superimposed Rivers.— If the elevation continues 
far enough, it causes the river to cut through the old flood 
plain deposits and to be superimposed on the hard rocks 
underneath. If the elevation proceeds slowly or by 
stages the river will begin to widen its new trench and 
develop a new flood plain at the lower level, into which 
the river may again cut by subsequent elevation. 

89. Terraces.— The remains of the flood plains left along the 
sides of the valley of a revived river form terraces, and the high- 
est ones are the oldest. Similar, but much smaller terraces may 



Fig. 67, Alluvial terraces formed by the uplift 
of an area and consequent downcutting by 
the stream. Terraces t t remnants of an 
early flood plain, t' t' of a later flood 
plain. F is the present plain. 

be formed by the ordinary down-cutting of the river in its first 
cycle. Terraces of this kind composed of sand, gravel, and silt, 
are called alluvial terraces and differ from the rock terraces 



GKOUND WATER AND RIVERS 



93 



which are caused by the harder and more resistant rocks pro- 
jecting as ledges or terraces along the sides of the valley. (See 
diagrams.) 




Fig. 68. Rock terraces, t t, formed by harder layers 
of rock projecting on the hillside. R, river, N, 
natural levee. 2, 3, 4, former positions of the 
river channel. The stream is now aggrading 
its valley and building up its flood plain. 

90. Reversed Drainage.— The lower or middle por- 
tion of a river basin may be elevated more rapidly than 
the upper portion. If it is elevated more rapidly than 
the river abrades its' channel, it will form a dam across 
the valley and a swamp, marsh, or lake will be formed 




Fig. 69. Rock terraces in the Alleghany plateau. The coal and under- 
lying clay beds disintegrate more rapidly than the intervening sand- 
stone beds which form projecting ledges or terraces. Springs emerge 
at the outcrop of the coal and form hillside bogs. 

above the dam, similar to that when the upper portion of 
the valley subsides faster than the lower portion. If the 
elevation continues the water may find an outlet over the 
divide at the headwaters, cut a deep channel, and thus re- 
verse the drainage of a large part of the river basin. The 



94 PHYSICAL GEOGRAPHY 

drainage is sometimes reversed by river piracy in the 
shifting of divides. (See sec. 93.) Some of the tributaries 
of the Ohio and Allegheny Rivers in Western Pennsylvania 
formerly flowed northward into what is now the Lake Erie 
basin. They were reversed by the glacier which came from 
the north. 

91. Antecedent River.— If the elevation of a land area takes 
place no faster than the river abrades its channel, then the 
stream will saw its way down through the rising land, which may- 
be a plain, a plateau, a mountain range, or even a mountain sys- 
tem, raised thus across the stream without diverting it from its 
original course. Such a stream is called an antecedent river as 
it was there before the elevation occurred. The Colorado River, 
where it flows through the Grand Canyon is an example. What 
other examples can you find? 

The elevation of a coast indented with bays and estuaries will 
produce engrafted rivers; that is, the rivers that previously flowed 
directly into the arm of the sea will become engrafted on one 
main stream flowing across the newly uplifted land. 

The coming of a continental glacier, such as once covered New 
York State and a large part of North America, would for the 
time being destroy all the rivers in the area covered, and after 
melting of the ice many of the rivers would begin to form new 
valleys and a somewhat complicated system of drainage would 
result. (Explained further in Chapters III and IV.) 

92. Water Gaps and Wind Gaps.— If a number of 
streams flow across the outcropping edges of hard and soft 
layers as shown in fig. 70, tributaries will develop on the 
soft layers at right angles to the main stream. These are 
called subsequent streams. It often happens that one of 
the main streams (as in fig. 71) deepens its channel faster 
than its neighbors. Its tributaries will then cut deeper 
and longer which in turn aids in deepening the main 
stream channel (how?) until in time its tributary (a) 
cuts back until it gains in succession the head waters of 
2, 3, 4, and 5. Another tributary (b) might in a similar 
way decapitate the captured branches of a as shown in 



GROUNDWATER AND RIVERS 



95 



fig. 71. The points where the stream cuts through the 
hard layers which now form ridges as at W. W. are called 
water gaps. The low depressions or notches in the ridges 
at PPP where streams formerly flowed are now called 
wind gaps and are utilized for highways in crossing the 
ridges. ( Study the Harrisburg, Pa. or the Harper 's Ferry, 
Va., topographic sheets for good examples of water gaps 
and wind gaps.) 




Fig. 70. An early stage in river piracy. AA and 
BB outcropping edges of hard rocks. 

93. Migration of Divides.— The shifting of the waters 
from one valley to another as described in the preceding 
paragraph causes a corresponding shift in the divides be- 
tween the valleys. This process of stream capture is 
known as river piracy. The shifting or migration of the 
divides goes on through youth and maturity until the 
divides become pretty well established in old age. Trace 
out on figs. 70 and 71 the divides before and after the shift- 
ing of the streams. 




96 



PHYSICAL GEOGRAPHY 



Examples of river piracy. Many examples of piracy and shift- 
ing divides may be traced out on the topographic contour maps 
with a little care. In New York good examples may be found on 
the Kaaterskill and Plaaterskill topographic sheets. 

In Pennsylvania the North Branch of the Susquehanna used 
to flow into the Delaware River through the Schuylkill valley by 
way of Wilkes-Barre. but a tributary of the West Branch of the 




Fig. 71. A later stage of river piracy than that 
illustrated in Fig. 70. Stream No. 1 has 
deepened its channel faster than 2, 3, 4, and 
5. Hence its tributaries have captured the 
upper portions of the other streams leaving 
wind gaps at P, P, P, P, and forming water 
gaps at W, W. Stream 1 is the pirate, streams 
2-6 have been beheaded. 



Susquehanna from Northumberland cut back into the softer 
shales faster than the old Susquehanna-Schuylkill could cut 
down the hard conglomerate over which it was flowing, so that 
the Schuylkill was decapitated at Wilkes-Barre and the upper 
portion drained into the Susquehanna. This change should be 
traced out on the geological map of Pennsylvania if one is at 
hand. 



GROUNDWATER AND RIVERS 



97 



study fig. 72 and see how the Shenandoah River captured the 
headwaters of Beaverdam Creek and left a wind gap at Snicker's 
Gap in the Blue Ridge. 





// 


TTc 




lii 






7^ 






tX. 




/S 


KL- 


, 


// 


A 







^# 




//or^/3 ^f^j^^^i r 




M 




-. S/f/c%efS Gap 


-^ 


^"^"^^^ 







Fig. 72. River Piracy. — B, present condition. A, probable condition ages 
ago. In A, Snicker's gap is a water gap through which Beaverdam 
Creek is flowing. In B the gap is a wind gap caused by the more rapid 
downcutting of the Shenandoah River enabling it to capture the headwaters 
of the other stream. Shenandoah River is the pirate, Beaverdam Creek 
has been beheaded. (After "Willis.) 

94. Streams in Arid Climates.— There are some forms 
of erosive action that are characteristic of dry or arid 
climates. In desert regions the little rain that falls 
descends in heavy thunder showers, separated often by 
long intervals, sometimes several months, sometimes * sev- 
eral years. The long intervals of dry weather cause the 
death of all vegetation and the heavy rains, falling on 
the bare soil, flow rapidly into and along the channel 
ways, called wadies in the Sahara desert, there cutting 
deep trenches with steep, frequently perpendicular sides. 

7 



98 PHYSICAL GEOGRAPHY 

The wady is frequently used as a highway by the trav- 
eller of the desert because he there finds some shade and 
protection from the hot scorching winds of the desert, and 
further any spring or water hole in the region is likely 




Fig. 72a. Arroyo, near Kingman, Arizona. A watercourse in an arid region 
subject to great floods from occasional cloud bursts, but dry most of the 
time. (D. T. McDougal.) 

to be there. It is because these wadies are so frequented 
by travellers that sometimes persons are drowned in them 
by the down-rushing flood following one of the sudden 
storjns. Similar deep gullies cut by the heavy rainfalls 
in the arid regions of the southwest United States are 
called arroyos. (See fig. 72a.) 

95. Playas.—In interior basins, that is, areas in which the 
rivers have no outlet to the sea, there are in places broad shal- 
low depressions, probably formed by the wind, that are covered 
with water after a heavy rain, but from which the water is 



GROUNDWATER AND RIVERS 99 

evaporated during the long, dry seasons. Such areas are called 
playas, a Spanish word meaning shore or strand. A playa of this 
kind occurs in Black Rock desert in Nevada, covering nearly a 
thousand square miles, which in the wet season is covered with 
water a few inches deep carried in by the Quinn River which flows 
into it. When the water is agitated, by high winds it becomes 
a lake of mud. During the summer season the water evaporates 
and the area is covered with a barren clay flat. Deeper depres- 
sions may form salt lakes. (See Chapter III.) 

REFERENCES 

Groundwater: 

Fuller, Underground waters of the U. S., Water Supply and 
Irrig. Papers No. 114 of the U. S. Geol. Survey. Many 
of the other bulletins in the same series contain ex- 
cellent articles on this subject and that of rivers. 
Nos. 44 and 67 are especially valuable. 

Schlichter, Crosby, et al, Underground Resources of Long 
Island, Professional Paper No. 44 U. S. Geol. Survey. 

Observations and Experiments on the Fluctuations in Level 
and the Rate of Movement of Groundwater, Bull. 
No. 5 Weather Bureau, U. S. Dept. of Agr. 

Hov6y, Celebrated American Caverns, Robert Clarke Co. 
Rivers: 

Russell, Rivers of North America, G. P. Putnam Sons, 1898. 

Davis, Rivers and Valleys of Pennsylvania, National Geog. 
Magazine, Vol. I, 1889. 

McGee, The Flood Plains of Rivers, Forum, April, 1891. 

Gannett, Profiles of Rivers, U. S. Water Supply and Irriga- 
tion Papers, No. 44. 

Gannett, The Flood of April, 1897 in the Lower Miss., Scot. 
Geog. Mag., Vol. 13, 1897. 

Powell, Exploration of the Colorado River, Washington, 1875. 

Penck, Valleys and Lakes of the Alps, 8th Int. Cong. Geog- 
raphy. 

Tarr, Watkins Glen and other Gorges of the Finger Lake 
Region, Pop. Sci. Monthly, May, 1906, Vol. 68, p. 387. 

MacDougal, The Delta of the Rio Colorado, Bull, Am. Geog. 
Soc, Jan., '06. 

Cole, Delta of the St. Clair River, Mich. Geol. Surv., Vol. IX, 
Pt. I. 



CHA*PTER III 
LAKES 

If one should take the delightful water trip from 
Duluth, at the head of Lake Superior, to Montreal, he 
would travel on four of the greatest lakes in the world 
and on five different rivers, yet all the way he w^ould fol- 
low the natural course of the water flowing from Duluth 
to the sea. He would actually travel over a part of one 
great river, but the lakes are so large as to obscure the 
importance of the comparatively small rivers flowing from 
lake to lake. The connecting rivers St. Mary's, St. 
Clair, Detroit and Niagara have been given separate names 
and commonly considered separate rivers. 

A lake is a body of comparatively still water, nearly 
surrounded by land. In some localities the smaller bodies 
of water are called ponds. In most of the lakes the water 
is fresh, in some it is salt, and in others alkaline. 

96. Relation to Rivers.— Sometimes a lake is the head of a 
river; more frequently it is a part of a river, occupying an ex- 
panded portion of the valley. Generally it is so much larger, 
wider and deeper than the river that the relation -of the two is 
not recognized. Nearly all lakes ha^e one or more streams flow- 
ing into them and one (rarely two) flowing out. In most lakes 
there is no perceptible current or flow of water as in the river. 

While in a moist climate a lake may be the head of a river, 
in an arid climate it may be the terminus of a river. In the 
latter case the lake will be salt or alkaline. 

97. Origin of Lake Basins.— Lakes are formed in 
many different ways: (1) Any depression (basin-like) 
which extends below the water table on a new land area in 

100 



LAKES 



101 



a moist climate will soon fill with water and form a lake. 
Such depressions may be due to inequalities on the sea 
bottom before the uplift, or made during the uplift, or 
may be made subsequently by the action of the wind, such 
as the playas already mentioned. (2) Lakes are formed 







Fig. 73. Delta of the Trinity River in Galveston Bay, Texas. The delta will 
in time extend across the bay and Turtle Bay will then be a lake and 
later a swamp. Compare with Salton Sink in Fia. 79 where the delta 
has cut off the end of the gulf. 

by rivers on the flood-plain (a) by cutting off meanders, 
the ox-bow lakes; or (b) by building up a natural levee 
across the mouth of a tributary, as on the lower Red 
River; or (c) by a tributary building an alluvial fan 
across the main stream. (3) A river may build a delta 
across a gulf or bay, thus forming a lake. (Figs. 73 and 79. ) 
(4) Lakes may be formed by the warping or twisting 
of the earth's crust (diastrophic lakes). Any bending of 



102 PHYSICAL GEOGRAPHY 

the crust that produces a basin-like depression will result 
in a lake in a moist climate. Lake Superior was formed 
in part at least in this way. 

The elevation of a portion of a stream valley would 
cause a lake in the valley above the elevation, providing 
the stream did not cut down its channel as fast as the 
elevation took place. 

(5) Lakes are formed by glaciers in several ways: (gla- 




FlG. 74. View from St. Regis Mt. in the Adirondack Mountains, showing 
numerous lakes of glacial origin. There is great irregularity in size 
and shape. Some are due to depressions in the glacial moraine deposit, 
others to depressions worn in the rock. (S. R. Stoddard.) 

ciers are described in chapter IV) (a). The moraine forms 
dams across a valley, especially on streams flowing 
towards the glacier. Such are the Finger Lakes in cen- 
tral New York, (b) The ice erodes depressions in the 
rock which fill with water and form lakes after the re- 
treat of the ice. (c) In a heavy moraine deposit there 
will be many kettle-like depressions which form lakes, 
(d) Where the water from the melting glacier flowed 
over the edge of a cliff it scooped out a basin at the 
foot similar to that at the bottom of Niagara Falls. Sev- 



LAKES 



103 



eral small lakes of this kind occur in the vicinity of Syra- 
cuse, New York. (See chapter IV). 
tzrts " »»io ' OS' I iz2* 




K2 



4/50* 



Fig. 75. Contour map of Crater Lake, Oregon. The numbers on the lake 
represent depth of water in feet. Numbers on the contour lines in- 
dicate feet above sea level. Maximum depth of water 1975. A caldera 
lake, due to sinking of the center of a volcanic mountain. (U. S. Geol. 
Survey. ) 

(6) Volcanoes may form lakes by streams of lava 
flowing across a valley and thus forming a rock dam; or 




Fig. 76. Cross section through Crater Lake. Section through the island 
shown on FiG. 75. (U. S. Geol, Survey.) 



104 



PHYSICAL GEOGRAPHY 



by the subsidence of the bottom of the crater forming a 
caldera or crater lake. (See Crater Lake sheet in the 
U. S. Topographic Atlas, No. 2). 

(7) Earthquakes sometimes cause the subsidence of 
considerable areas which fill with water and form lakes. 
Several such lakes were formed near the mouth of the 
Ohio River in southeastern Missouri in 1811. Near the 




FiQ. 77. Reelsfoot Lake. Teim., formed by subsidence of the area during 
the earthquake of 1811. Stumps are the remains of the forest which was 
submerged at that time. (M. L. Fuller.) 

village of Lone Pine in Owen Valley, California, a lake 
was formed by an earthquake shock in 1872. Pig. 77 shows 
a lake formed by the Mississippi Valley earthquake in 1811. 
(See Sec. 239.) 

(8) Landslides and avalanches sometimes form dams 
across valleys, causing lakes. In 1893 a landslide esti- 
mated to contain about 800,000,000 tons of rock fell across 



LAKES 



105 



one of the tributaries of the Ganges River, and built a dam 
nearly a thousand feet deep, which caused the water to 
back up the Valley about four miles, and form a lake of 
that length. 

(9) Small lakes are sometimes produced by beavers 
building dams across the stream. Many of the *' Beaver 
Meadows'' through the northern United States are the 




i'iu. id. iieaver Dam seen from below. The beavers build the obstruction in 
a stream channel which produces a lake. (U. S. Biological Survey.) 

remnants of beaver dams now partly or wholly filled by 
vegetation. (Fig. 78.) Sometimes growing vegetation be- 
comes sufficiently dense to obstruct the stream channel and 
produce a lake. 

(10) Small lakes are sometimes formed by the chemical 
action of the groundwater dissolving and carrying away 



106 



PHYSICAL GEOGKAPHY 



large quantities of rock material, leaving depressions that 
fill with water and form lakes. In limestone regions such 
depressions frequently have an opening at the bottom into 
a cave and are called sink-holes. When the hole at the 
bottom of the depression becomes stopped so that the 
water does not get through, the depression fills with 
water, forming a small lake. In the limestone regions of 
Kentucky and Indiana, where surface water is scarce, the 
farmers frequently stop the opening in the bottom of the 
sinks with clay in order to hold the water for the stock. 
(See fig. 22, Sec. 49.) 




Fig. 78a. Natural rock dam on Pucaswa River, Ontario. View looking 
up stream. The massive rocks in the channel form part of a dyke of 
harder material than the rocks on either side. Erosion of the softer 
material has formed a pool or lake above the dyke and a shallow 
channel below. 

(11) Lakes or ponds of limited extent are sometimes 
formed along stream courses where the stream crosses the 



LAKES 



107 






f»\*'n't 






H P tJ,\A 







Fig. 79. Relief map Salton sink before it was flooded. The gulf of California 
at one time extended northwest beyond Indio. The distributary channels 
indicate the delta deposit. A large part of the low area north of the 
delta is now (1907) covered with water. The S. P. R. R. was moved 
northeast to the higher land. Compare with FiG. 73, which shows an 
earlier, stage of a similar phenomenon on a smaller scale. 



108 



PHYSICAL GEOGRAPHY 



outcropping edge of a layer of hard rock that is both over- 
lain and underlain by softer layers. The action of the 
weather and water wears away the softer rock faster than 
the harder which then forms an obstruction or natural 
dam across the stream. (Fig. 78a). 

98. Some Examples of the Different Classes of Lakes: — 
(1) Great Salt Lake and many of the other lakes in the Great 
Basin area are remnants of lakes of the first class. Black Rock 
desert on the Quin River in Nevada is a good example of the 
playa, which is a lake only in the wet season, and a mud-covered 
plain the remainder of the year. 




Fig. 80. Beach of Salton Sea. Ancient beach at right. Present (1907) 
beach at left. Intermediate stage in the center. The beach at the right 
is 22 feet above sea level. (D. T. McDougal.) 

(2) Good examples of the second class occur on the flood- 
plains of the lower Mississippi River. Study the maps of the 
Mississippi River Commission. (See fig. 57). 

(3) Salton Sink in Southern California was at one time part 
of the Gulf of California. The Colorado River fiowed into this 
gulf at Yuma and depositing the great mass of material eroded 
from the canyons, it built out the enormous delta which in time 
extended entirely across the gulf, thus cutting off the northern 



LAKES 



109 



end and forming a great salt lake. The river established a chan- 
nel on the south side of the delta, flowing into the open gulf, but 
sometimes during high floods part of the water would overflow 
into the sink on the north side. (Figs. 79, 80 and 81). 

The rainfall was so light on this area and the air so dry and 
warm that evaporation took place rapidly; the lake dried up and 
the lake bottom, formerly the gulf bottom, became a dry plain in 
part covered with salt and lying, at the lowest point, 287 feet 
below the sea level. 

The soil covering this low plain is very productive where 
there is sufficient water, so a few years ago an irrigating ditch 
was dug from the river across the delta plain and the country 
laid out in farms. 

Everything prospered at first, but during a high flood the 
river became unmanageable like an untamed horse that has sud- 




FlG. 81. New River at Calexico, February, 11)()7. Some of the houses 
of Mexical visible on the bluff at the left, the remainder of the town 
was swept away by the river. (D. T. McDougal.) 

denly discovered its power. The water flowing through the ditch 
was given a much lower base than the main river, hence it be- 
came at once a revived river and began to cut down and degrade 



110 



PHYSICAL GEOGRAPHY 



its channel. This cutting began during a flood when the open- 
ing became so large that the engineers were unable to check it, 
and nearly all the water of the river continued to flow through 
the irrigating channel. 

Since this inflow of waters threatened to flood all the de- 
pressed area, the railway company whose road was being des- 
troyed, joined forces with the irrigation company and at an 
enormous expense completed a dam across the ditch at the river. 




Fig. 82. Lakes Thun and Brienz were formerly one which was severed at 
Interlaken by mass of sediment carried in by the two streams shown on 
the sketch. Interlaken is built on the dividing delta. 

but in a short time the river cut a new channel around the dam 
and again (January, 1907) poured its flood of waters into the 
sink. Another dam was completed and the river again turned 
into its former channel. 

A great cataract, nearly a mile wide and 90 to 100 feet high, 
was formed on the lower course of the irrigating river and be- 
fore the break was closed was cutting its way back at the rate 
of one-third of a mile per day. 

This is one of the most Sifflcult and serious problems that 
has ever confronted irrigating engineers. If they do not suc- 
ceed in keeping the river in its old channel, what will be the re- 
sult in the Salton basin? Suppose they flnd no means of stopping 
the receding waterfall, what will be the result when it reaches 
the great Laguna dam above Yuma? This dam was constructed 



LAKES 



111 



across the Colorado River at a cost of a million dollars, for pur- 
poses of irrigation. With no interference from man, what would 
finally become of the cataract? Why should this cataract recede 
so much faster than Niagara Falls? 

The Alpine lakes, Thun and Brienz, in Switzerland, were at 
one time united in a single lake. Two streams flowing in from 
opposite sides formed deltas which extended into the lake until 
they met in the middle and thus cut the lake in two. The town 
of Interlaken (meaning, between the lakes) is built on the 
dividing delta. (Fig. 82.) 

99. The Great Laurentian Lakes.— Upon the north- 
ern boundary of the United States is a chain of the lar- 
gest fresh-water lakes in the world. They formed a use- 
ful and important highway to the Indian and the pioneer, 
and are now serving in the same way for a great and in- 
creasing inland commerce. The agricultural products 
and mineral wealth of the great west find their way in 
large quantities over these lakes to the eastern markets, 
while products of the eastern coal fields and the great 
factories pass westward in return. The dimensions of the 
Great Lakes are shown in the following tabulation: 

The Great Laurentian Lakes 





Lake 


Lake 


Lake 


Lake 


Lake 




Ontario 


Erie 


Huron 


Michigan 


Superior 


Area in square miles 


7,240 


9,960 


17,400 


20,200 


31,200 


Length of shore line 


800 


590 


735 


875 


1,300 


Maximum depth 


738 


210 


730 


870 


1.008 


Average depth 


300 


70 


210 


335 


478 


Depth below sea level 


491 




149 


289 


406 


Elevation of surface 












above sea level 


247 


573 


581 


581 


602 



100. Salt lakes are formed in an interior basin which 
has a moderate rainfall, sufficient at least to cause some 
of the water flowing from the surrounding highlands to 



112 



PHYSICAL GEOGRAPHY 



extend as far as the lowest depression in the basin. The 
area of such a lake fluctuates with the seasons and the 
climate, rising in the wet season and sinking in the dry- 
season. If the climate becomes more arid, the lake will 



SQd 



/dOO 




tm 



Fig. 83. Diagram showing comparative depth of the Great Lakes. 



decrease in size and may even disappear, leaving a deposit 
of salt. If the climate becomes more moist, the lake will 
increase in size until it fills the basin and overflows, when 
the salt will be carried out and the lake become fresh. 

Great Salt LaTce in Utah is growing smaller and salt is being 
deposited around the borders at present, but at one time it 
covered a larger area in the Great Interior Basin and overflowed 
to the north. 

In early geological times there was a dry climate in central 
New York, probably as dry as that in Utah to-day, as shown by 
the great beds of rock salt which were formed at that time. 

There are fresh water lakes in the Great Interior Basin of 
the United States and other dry regions, but they have an out- 
let. Salt lakes are those which have no outlet, that is, they are 
the last basin into which the water flows and from which it 
escapes only by evaporation. The ocean is the greatest body of 
salt water and it is the basin into which most of the great rivers 
of the world empty. 

Salinas are salt plains, sometimes marshes, sometimes 
dry plains, that were probably salt lakes but which have 
dried out from change of climate or other cause. Some 
of them are marshes at one season of the year and dry 
salt plains at other seasons. 



LAKES 113 

Alkaline lakes are formed similarly to salt lakes when 
the inflowing streams carry more alkalies than salt in so- 
lution. (See fig. 84.) 




Fig. 84. An alkali lake on the Laramie plain 18 miles west of Laramie. 
This lake has no outlet. The temporary streams flowing into the 
lake after heavy rains carry some alkali which accumulates from 
year to year as the water escapes by evaporation. (U. G. Cornell) 

101. Fluctuations of Lake Levels.— The level of some 
lakes varies greatly from time to time due to one or more 
of several causes, (1) One of the most common and 
noticeable changes in level is due to the change in season 
from wet to dry. During the wet season the water level 
in the lake rises from a few inches to many feet in differ- 
ent lakes. During the dry season the level sinks a cor- 
responding distance due to the greater evaporation, and 
in arid districts the water may even evaporate entirely 
from lakes that are of considerable size in a wet season. 

(2) Prevailing winds sometimes produce a marked 
effect. With a strong west wind continuing for several 
days, the water has been known to rise as much as fifteen 
feet at the east end of Lake Erie, with a somewhat cor- 
responding depression at the west end. 



114 PHYSICAL GEOGRAPHY 

(3) Difference in atmospheric pressure may cause a 
temporary subsidence of the surface at one place, and ele- 
vation at another. Thus a marked high pressure area at 
the west end of Lake Erie might cause a sinking of the lake 
surface there and a rise of the surface at the east end. This 
rising and sinking of the surface is called a seiche. During 
a seiche the water rises or sinks from a few inches to a few 
feet. (See sees 289 and 296.) 

102. How Lakes Disappear.— As already stated lakes 
on the upland are signs of youthful topography. They 
are short-lived in comparison with rivers, plains, moun- 
tains and other natural features. Lakes may disappear 
in different ways: (1) If the stream that drains the lake 
cuts its channel deep enough it will drain the water from 
the lake basin. The bottom of Lake Erie is at about the 
same level as the bottom of Niagara Falls; should the 
falls recede as far as Buffalo, Lake Erie would be drained 
and there would be a river flowing across what is now the 
lake bed. 

(2) The streams flowing into lakes carry sediment 
which is deposited in deltas and distributed over the lake 
bottom until the basin is filled and the lake disappears. 
This is one of the most active agencies in destroying lakes. 

Deltas should be studied in the vicinity of the school. Where 
permanent lakes are absent, deltas may be studied in pools formed 
by rains. (See fig. 82.) 

(3) Many lakes are slowly being destroyed by animal 
and vegetable matter which accumulates in sufficient quan- 
tities to fill the basin. Small molluscs, commonly known 
as periwinkles, grow in great numbers in some of the small 
lakes and as they die their shells accumulate on the lake 
bed forming bodies of shell marl which occur in some 
places fifty feet or more in depth. Marl is frequently 
composed partly of plant remains. Vegetation grows 



LAKES 



115 



on the bottom of lakes and around the margin. In the 
small lakes in cold climates a plant known as the sphag- 
num or pea^ plant grows in great luxuriance even on top 
of the water; the remains of this vegetation accumulates 
on the lake bed as peat, or muck until it finally fills the 
lake basin. Frequently the filling of the basin is due to 




FlQ, 85. Upper Avisablc lake in the Adiroiuhiek Aiouutains. The laKe 
is being filled by vegetable matter. The shrubby growth at the sides 
is on the part completely filled. The shrub area is extending towards 
the center of the lake. The bordering forests are crowding upon the 
shrub area. Notice the zonal arrangement of the vegetable growth. 
Part of the bordering flat is a quaking bog. (S. R. Stoddard.) 



the combined action of these two agencies. Many of the 
peat and muck areas (vegetable) are underlain by marl 
(animal) deposits. Hundreds of small lakes in New- 
York and elsewhere have been destroyed in this way. (Fig. 
85.) 

(4) Lakes may be destroyed by volcanic action in one 



116 



PHYSICAL GEOGRAPHY 



of two ways: (a) by material ejected from a volcano fill- 
ing the basin; or (b) as in the case of the lake on Mount 
Pelee, Martinique, the eruption takes place underneath 
the lake and blows it out of existence. 

(5) Winds also assist in filling lake basins by blowing 
in sand and dust from the surrounding area. 







H 




, 


^^,. 


^^S 


^S 


^^^^^3 


t|||!li||| 


^^i^l^^P 


1 '^ '^ 


C 




1 ■■- : 







Gilbert's map of glacial lake Iroquois 

Fig. 86. The dotted line on the shaded area shows the present boundary 

of Lake Ontario. The greater lake Iroquois drained through the Mohawk 

valley because the glacier then filled the St. Lawrence valley and pre^ 

vented the water from flowing through that valley as it does at present. 

103. Fossil Lakes.— Areas formerly covered by lakes that 
have been filled or drained by some means, are classed as fos- 
sil lakes, and they may be recognized by the following marks: 
(1) By characteristic shore features or markings, such as cobble 
or gravel beaches, or sand, gravel or wave-cut terraces. (Des- 
cribed in Chapter VI on Shore Features) (2) By characteristic 
lake-bed deposits such as peat, marl, diatomaceous earth, (see 
sec. 105) and bog iron ore. Lake Iroquois at the close of the 
glacial period, covered a large area south and east of Lake 
Ontario, an area now bordered by shore features and covered 
with lake-bed deposits.* (Fig. 86.) 

* Fossil Lake Passaic in New Jersey is described in the Annual Report 
New Jersey Geological Survey, 1893, and fossil Lake Agassiz, northwest of 
Lake Superior, is described in a large monograph of the U. S. Geological 
Survey. 



LAKES 



117 




Fig. 87. View in a gravel quarry on Fossil. Lake, Iroquois beach near Wolf 
Street in Syracuse. Notice the characteristic beach structure in the 
gravel, the slope of the layers from right to left. The island shore line 
was on the right. 

104. Life in Lakes and Rivers.— All the rivers and 
most of the lakes except the salt and alkaline lakes contain 
many forms of animal and vegetable life. The animal 
life is probably more prolific in the larger lakes and the 
vegetable life more abundant in the smaller and shallower 
lakes. As already stated many of the smaller lakes ter- 
minate by being filled with the accumulated remains of 
the animals and plants. 

Some animals, such as eels and salmon spend part of 
their existence in salt water and part in fresh ; but with a 
few exceptions of this kind, the life, both animal and plant, 
in the rivers and lakes is decidedly different from that in 
the sea. The life of the sea is more varied and locally more 
prolific than in the lakes. 

105. Diatoms.— There is one class of exceedingly small., 



118 



PHYSICAL GEOGRAPHY 



microscopic plants known as diatoms, that are found abundant- 
ly in both the lakes an^ the ocean. The diatoms are composed 
of opal, that is, silica, combined with water. So small are these 
plants that a German scientist, Ehrenberg, has estimated that 
there are about four billions of them in a cubic inch. Yet so 




Fig. 88. Micro-drawing of diatoms from the mountains at Lompoc, Cal. 
(W. F. Prouty.) 

numerous are they that in some places they form deposits many 
hundreds, even thousands of feet in thickness. The material 
composed of diatoms is known as tripoli or diatomaceoufs earth. 
It is so light and porous that it floats on water. It is used as a 
polishing powder, as a filler for soaps, as an absorbent for nitro- 



LAKES 



119 



glycerine in making dynamite, and as fire-proof material in 
buildings. A slippery brown material that covers the stones in 
the brook in many places is composed of diatoms. The large 
deposits of diatoms in California are in some places snow white, 
in others colored by the impurities mingled with them. It is 
estimated that an area of more than 10,000,000 square miles of 
the sea bottom is covered with diatomaceous deposits. (See 
figs 88 and 89.) 




Fig. 89. View of mountains formed by a white diatom deposit at Lompoc, OaL 
The deposit composed entirely of diatom remains is more than 1000 feet 
thick, and covers an area of many square miles. 



106. Functions of Lakes.— Lakes have a number of 
important functions which are directly or indirectly of 
commercial importance to man in his varied industries. 
(1) They serve as a regulator of floods. A river like the 
St. Lawrence with many large lakes in its course is never 
troubled with such destructive floods as visit the Ohio 



120 PHYSICAL GEOGRAPHY 

River which has almost no lakes in its basin. The rain 
that falls in the upper St. Lawrence basin flows into the 
Great Lakes where it spreads out over thousands of 
square miles of lake surface with almost no effect on the 
Niagara or St. Lawrence rivers below, while the rain that 
falls in the upper Ohio Valley, having no lakes in which 
to spread, runs rapidly into the river channels causing 
great floods. In flood season the Ohio River has been 
known to rise 50 to 60 feet above low water while a rise 
of half as many inches is rare in the St. Lawrence. (2) 
They form a valuable water supply for cities. (3) They 
serve as highways of navigation. The commerce on the 
lakes on our northern boundary now reaches quite ex- 
tensive proportions. (4) They form valuable fishing 
grounds. (5) The larger lakes form excellent sites for 
cities and the smaller ones for summer resorts. (6) They 
temper the climate in their vicinity. (7) They furnish 
a constant and steady supply of water to the rivers flow- 
ing from them. They serve as settling tanks for the 
rivers. The waters flow in muddy, the mud settles and 
the stream flows out clear. 

107. Life History of Lakes.— Like other natural 
phenomena, a lake has a period of growth, maturity, de- 
cline, old age, and death, which may be termed its life- 
history. This is not uniform but varies with different 
classes of lakes. The greatest variation is between lakes 
in dry and those in moist climates. In moist climates the 
average life of the lake is not long in comparison with the 
life of a river or mountain, but much longer than the life 
of any animal or plant. 

Lakes may come into existence in any one of the sev- 
eral ways mentioned in the preceding pages. With the 
exception of a very few lakes that have been blown up by 
volcanic explosions, they are destroyed slowly by the com- 



LAKES 121 

bined action of the different agencies previously described. 
When a lake basin is filled, the lake disappears as a body 
of water and the streams meander across the fertile plain 
formed of lake deposits. In the course of time the stream 
leading away from the old lake basin will lower its chan- 
nel, because it now carries sediment which was formerly 
deposited in the lake. This will cause the streams flowing 
over the plain of the lake-filled basin, to quicken their 
velocity and hence erode the soft materials, finally carry- 
ing away all the deposits that filled the lake, thus destroy- 
ing the last vestiges. The streams on the Fargo, North 
Dakota, topographic sheet are flowing over a lake-filled 
plain and are- just beginning the work of carrying away 
the sediment. (See figs. 47 and 48.) 

The lake is thus seen to be an incident in the life of a 
river which deposits in the lake bed the load of sediment 
it is carrying from the mountains to the sea. At a later 
period after having filled the lake basin, it again takes up 
the sediment and carries it on to the next stopping place 
and finally to the sea, the largest lake of all. 

Many small lakes have a different history from the above, 
because they have scarcely any sediment carried into them; in 
fact, there are many small lakes that have no surface streams 
flowing into them and either no stream or else a very small, 
sluggish stream flowing out. (See fig. 90). Such lakes will be 
filled in tim^ by the remains of plants and animals that grow and 
die in the lake. Such a lake forms first a swamp which later 
becomes solid and forms a meadow or vly as it is called in the 
region of the Adirondacks. There are no deltas, beaches, or 
evidences of wave action in most of these small muck-and-marl- 
filled lakes. 

108. Lakes in Arid Regions.— In arid regions the life 
history of the lake is somewhat different, being in general 
longer and more complex than in a humid region. In an 
enclosed basin-area the lowest depressions will have no 



122 PHYSICAL GEOGRAPHY 

outlet and hence cannot be destroyed by draining. The 
sediments and salts carried in and deposited, raise the 
lake-bed, but this in turn raises the level of the water and 




Fig. 90. Glacial lake near Pilot Harbor, Ontario. The end of 
the lake at the left has been filled and is now covered with 
swamp grass and shrubs. On the middle portion the forest 
grows to the water's edge. The lake will in time be filled 
with vegetable matter and form a meadow. 

causes it to spread out over a greater area. But increased 
area means increased evaporation and hence the lake in- 
creases or diminishes as the case may be, until the evapo- 
ration equals the inflow. The lake will be subject to many 
vicissitudes with change of climate, until it finally ends 
in one of two ways: (a) aridity may increase until the 
lake dries up and disappears as such until there is another 
change in climate; or (b) the humidity may increase un- 
til the entire basin fills up and overflows when it ends as 
any other lake in the humid region. Lakes may become 
salt in a humid region by being depressed below sea level 
where the sea water has access to the basin, when it be- 
comes for a time an arm of the sea. 



LAKES 123 

Following the glacial period, Lake Champlain was an arm 
of the sea which extended up the St. Lawrence Valley and filled 
the Champlain Valley 130 feet above the present level of the 
lake. A later elevation of the land drained the lake to its 
present level and the fresh water streams flowing into and 
through it carried the salt to the sea, thus changing it to a fresh 
water lake. 

109. Swamps and Marshes.— Swamps are for the 
most part closely associated with lakes and rivers and 
are likewise a kind of connecting link between the water 
and land areas. There are both fresh water and marine 
marshes.* The fresh water swamps may be conveniently 
divided into river, lake, and upland swamps. 

The river swamps may be divided into terrace and 
flood-plain swamps. The terrace swamps, sometimes 
called hillside bogs, are formed by the outcrop on the hill- 
side of a bed of clay, shale, or similar rock that causes a 
continual seepage of the groundwater into the clay soil on 
the surface. They are abundant in the bituminous coal 
fields of the Appalachian plateau where they are caused 
by the outcrop of the clay beds that underlie the coal 
seams. The flood plain swamps are in great numbers on 
nearly every river flood plain and delta as already de- 
scribed. (See figs. 58 and 69.) 

Lacustrine or lake swamps are of two classes: those 
formed on the lake margin caused by a rise and overflow 
of the lake or by the elevation to the surface of all or part 
of the lake bottom through the accumulation of vegetable 
or animal remains, such as the shell deposits which form 
the marl. The second class, known as quaking hogs, is 
caused in the final stages of lake-filling by vegetation, 
when the floating plants on the surface join those growing 
out from the shore, forming a continuous surface of vege- 

* There is a tendency at present to use the word "swamp"' for the fresh 
water forms and "marshes" for the marine. 



124 



PHYSICAL GEOGRAPHY 



tation across the remnant of lake water underneath. The 
climbing hog is formed by the vegetation drawing the 
water by capillary attraction above the level of the lake, 




Fig. 91. Climbing bog, L, lake. B, bog. PP, peat plant growing on float- 
ing vegetable remains. M, muck accumulating on the bottom. C, 
climbing bog. (After Shaler.) 

extending the bog above and beyond the former lake- 
shore. (See figs. 91 and 92.) 

Upland swamps may be formed on clay soils by the 
accumulation of plant remains which prevent the rapid 
drying out of the moisture while the underlying clay re- 
tards its descent as groundwater. Such swamps some- 
times build up many feet above the level of the surround- 
ing flat on which they are located, and at times after 




Fig. 92. Bog and swamp formed by lake filling. DD, diatomaceous de- 
posit. II, iron ore. PP, peat. S, swamp. B, quaking bog. C, 
climbing bog. W, remnant of lake not yet filled. (After Shaler.) 

heavy rains, have been known to burst and flood the ad- 
joining areas with a mass of black mud or muck formed 
by the decaying vegetation. 

110. Salt Marshes.— Besides the fresh water swamps 
there are vast areas of salt marshes along the seaboard. 
(See chapter on Shore Lines). Professor Shaler esti- 
mated that there are at least 350,000 acres of marsh land 



LAKES 



125 



between New York City and Portland and that 200,000 
acres of this could be reclaimed, drained, and made into 
agricultural land that would have a value of $40,000,000. 

111. Economic Features of Swamps and Marshes.— Swamps 
and marshes are not entirely barren stretches. Among the 
many economic products from them might be named the fol- 




FlG. 93. Marl deposit in former lake bed at Wariiprs, N. Y. 
now being quarried for use in the manufacture of Portland 
cement. (S. H. Ludlow.) 

lowing: rich agricultural land after drainage, timber, peat, cran- 
berries, tripoli, marl, phosphates, bog iron ore, regulation of 
streams, game. What other economic products or features of 
swamps can you name? Enumerate some of the undesirable 
features of swamps. 



FALLS AND RAPIDS 



112. A stream flowing from a lake at one level to 
another at a lower level, or in flowing from an upland to 
a lowland valley will have rapids or falls at the abrupt 
descents in the course. Lake Superior is 21 feet higher 



126 PHYSICAL GEOGRAPHY 

than Lake Huron and the St. Mary's River which con- 
nects the two descends this distance over the rapids at 
the very outlet of Lake Superior. Lake Erie is 326 feet 
above Lake Ontario and the connecting river descends 
half of this distance suddenly at the falls in the Niagara 
River and the other half mostly in the rapids above and 
below the falls. 

Falls or rapids may come into existence in a stream 
where its course leads it over a cliff or steep slope, or they 
may develop on a stream course which was at first uni- 
form, providing there are one or more hard layers of rock 
separated by softer layers all lying horizontal or inclined 
up stream. Where the stream flows across the outcrop- 
ping layers of the ledges, the underlying softer one will 
be eroded more rapidly than the overlying harder one 
which in time is undermined until the overhanging por- 
tion breaks down of its own weight and thus causes the 
falls to move slowly up stream. This process will con- 
tinue until the grade of the stream brings the bottom of 
the channel above the top of the soft layer, when the falls 
will change to rapids, and the rapids recede until they 
are graded and thus disappear. 

113. Niagara Falls.— The Niagara River flows over 
the Lake Erie plain from the lake to the falls where it 
drops 160 feet into the gorge through which it flows un- 
til it emerges on the Lake Ontario plain at Lewiston, 
seven miles below. Above the falls the river flows over 
the Niagara limestone which also forms the cap rock on 
both sides of the gorge. Underlying the hard limestone 
is a bed of softer shales which is more readily eroded by 
the water than the hard limestone at the top. The water 
plunging into the pool at the base of the falls wears away 
the softer shales and leaves the limestone projecting as 
an overhanging ledge until it breaks off by its own weight. 



LAKES 



127 



causing the falls to recede the width of the fallen mass. 
The repetition of the process has caused the falls to move 
back, thus lengthening the gorge more than three hundred 
feet since the first measurement was taken in 1842. (Study 
figures 94 and 95, and the U. S. topographic map of Niagara 
Falls.) 




Fig. 94. Part of Niagara Falls and gorge. View from Goat Island. Notice 
the shallowness of the water near the American shore. (E. R. Smith.) 

The continuation of this process in the past has 
caused the river to cut the gorge all the way, about seven 
miles, from Lewiston to the present position, and if the 
movement continues, the falls will eventually be carried 
back to Lake Erie and the lake will be drained. How- 
ever, it is probable that the falls will change to rapids 
and new falls develop on higher beds before Lake Erie 
is drained. 

If the rate was uniform in the past, as at present, (about 



128 



PHYSICAL GEOGRAPHY 



five feet per year), how old are the falls now? How long will 
it be until they reach Lake Erie at the same rate? (See the 
contour map sheets of this region for distances.) Can you infer 
from the diagrams showing the position of the rocks, whether 




Pia. 95. Vertical section at Niagara Falls. The softer shales underneath 
are worn away by the water. The overlying hard limestone breaks oflf 
and is carried down the gorge. The repetition of this process causes 
a recession of the falls and the lengthening of the gorge. (After 
Gilbert. ) 

the rate of recession has been increasing or decreasing, and 
how it will be in the future? What difference would it make 
if the Niagara limestone were near the surface of Lake Erie in- 
stead of below the bed of the lake? 



LAKES 



129 



The rapids on the St. Mary's River will recede in time until 
the surface of Lake Superior is lowered to, or near the level of 
Lake Huron, but the recession will be very slow, because the 
water flowing out of the lake carries little or no sediment and 
hence no graving tools to cut away the rock over which it is 
flowing. The rock being a hard sandstone is not soluble, so that 
it cannot be carried away in solution. 

114. Falls Formed by Frost.— At the Stone Quarry 
Falls, near Manlius, New York, there is a hard limestone 









Fig. 96. Stone Quarry Falls at Manlius, N. Y. The falls pro- 
ject into the gorge with a recession on each side. The dis- 
integration by the frost on each side of the falls is more 
rapid than the wear of the rocks by the water. 

rock at the top of the falls underlain by shaly limestone 
and shale similar to Niagara Falls, except that the lime- 
stone is thicker in proportion to the shale. Here the water 
does not undermine the limestone by wearing away the 
softer shale but the shale projects out beyond the lime- 
stone and forms a rounded prominence at the head of the 
gorge over which the water descends by successive stages 
9 



130 



PHYSICAL GEOGRAPHY 



from layer to layer. The middle and foot of the falls 
project several feet beyond the top instead of the reverse 
as at Niagara Falls. (See fig. 96.) 

At the Stone Quarry Falls, the gorge is wider than 
the falls and there is a recession at the head of the gorge 




Fig. 97. View in Havana Glen, N. Y., illustrating the effect of joint planes 
on the direction of the stream and bordering cliffs. Near the middle of 
the picture the stream makes a turn at right angles in changing from one 
system of joint planes to another. 

on each side of the projection under the falls. The ex- 
planation is found in the relatively greater work of the 
frost. Under the falls the rock is protected by the run- 
ning water which does not freeze; at the sides, which are 
moistened by the spray, the frost splits off fragments, 
causing the greater recession. Instead of erosion under 
the falls there is even deposition at times of some carbon- 



LAKES 



131 




132 



PHYSICAL GEOGRAPHY 



ate of lime from that held in solution by the water in the 
stream. 

In some places waterfalls recede by the splitting off 
of blocks along the vertical joint planes. Joint planes are 
natural planes of parting, generally vertical, that intersect 
all rocks, but are especially prominent in some sedimentary 
formations. Where the bottom of the cliff is eroded, the 
overhanging portions break away along these planes and 
fall in huge blocks leaving the smooth vertical walls of the 
joint plane. These sometimes influence the direction of a 
stream at the falls. (See fig. 97) 




Fig. 99. Montour Falls near Watkins Glen, N. Y. The water flows over a 
prominence with a depression on each side formed mainly by action of the 
frost. What causes the vertical face at the base of the falls? 



At Barnett Falls, Vermont, as shown in fig. 98 the 
stream is being turned from its present course by the 
natural cleavage planes in the rocks. There are many ex- 



LAKES 



133 



amples of falls of this kind in AVatkins and Havana Glens, 
and elsewhere in the Finger Lake region of New York. 

Other natural planes of parting in the rock sometimes 
have an effect similar to that of joint planes. (See fig. 98.) 

How many of the waterfalls that you have seen belong 
to the Niagara type? Can you explain the origin of any 
of the others that are of a different type? Montour Falls 
near Watkins Glen belong to the Stone Quarry type. 
(See fig. 99.) 




Fia. 100. Tinker Falls near Tully, N, Y. The water falls from 
the hard Tully limestone upon the softer Hamilton shales. The 
deep recession back of the falls, 30 feet or more, is due to the 
action of frost and other weathering agencies. 

Falls and rapids may be formed in other ways than 
those described above, but the ones mentioned are typical 
of hundreds of similar examples scattered over New York 
and other parts of the United States. 

Waterfalls are more numerous in the Northern United 
States than in the central and southern portions, because of the 
action of the glacier that formerly covered the area. The gla- 
cier by deepening and widening many of the larger valleys and 



134 



PHYSICAL GEOGRAPHY 



removing the talus material at the bottom of the hill slopes, 
caused the smaller tributary streams to enter the main valleys 
over cliffs, thus producing cataracts. These are characterized 
as hanging valleys (see sec. 136). In some instances the trib- 
utary valleys were filled by glacial debris causing the stream to 
form a new channel. The new course of the stream frequently 
led it over a cliff, resulting in a waterfall. 



/ Li]-me»itQ'nel 




Fia. 101. Vertical section at Tinkers Falls, N. Y. The soft shale bed near 
the middle of the section disintegrates more rapidly than the other 
rocks and thus makes the deep notch back underneath the falls. During 
the dry season the stream of clear water falls at L and does little 
eroding. During high water, the larger stream carrying sediment 
falls at H wearing away the lower end of the talus, even wearing a 
basin in the bed rock at H. 



Reaches. On many rivers like the Genesee in New York 
and the Yellowstone in the National Park there are several 
waterfalls separated by more or less graded reaches similar 



LAKES 135 

to those which separate the rapids at a later period. (See 
sec. 69). These reaches may vary in length from a few 
inches to several miles. 

115. Economic Importance of Waterfalls.— The solar 
energy that lifts the water from the sea into the atmos- 
phere is stored in the raindrops and this is concentrated 
at the waterfalls in such a way that man can utilize it to 
turn the wheels of his machinery and thus turn it into 
mechanical energy, heat, or light. 

In the early settlements of North America the vicinity 
of the waterfalls was the point first selected by the 
pioneer for his home and especially for his villages and 
towns, because here he found the energy to run the mills, 
to grind his corn and saw his lumber. Later when steam 
power was discovered and utilized and the great beds of 
coal were found in Pennsylvania and elsewhere, steam 
was used to run the factories and the waterwheels were 
neglected in many places. 

Recent improvements in electrical appliances, by which the 
energy in the waterfall can be cheaply transported on metallic 
wires long distances and turned into mechanical power, heat, 
or light as desired, emphasize again the importance of water- 
falls, poetically called "White Coal," and the power is now being 
used in hundreds of places. Energy from Niagara Falls is 
carried by wire into central New York to light the cities, run 
the cars, and furnish power for some of the factories. What 
other falls can you name that are utilized in a similar way? 

116. The Fall Line.— Along the Atlantic sea coast 
is a coastal plain of varying width which is underlain by 
beds of sand, clay, and gravel that are much softer than 
the crystalline metamorphic rocks in the old Piedmont 
belt against which they lie. The rivers flowing from the 
hard rocks of the upland to the softer deposits of the 
coastal plain, form falls or rapids at the point of contact. 
Many of the streams have a deep navigable channel 



136 



PHYSICAL GEOGRAPHY 



across the plain from the falls to the sea. Naturally these 
falls were among the first points selected by the early set- 
tlers as sites for their villages which later grew into the 
most important cities along the Eastern United States. 




Pig. 102. Fall line on a small stream. The stream flows over 
a bed of hard limestone in the midst of softer rocks. The 
falls visible in the background are of the Niagara type and 
have receded about 200 feet forming the gorge through 
which the stream is flowing. The falls will soon change 
to rapids. Why? (E. R. Smith.) 



Besides the useful water power obtained from the falls, 
other reasons for their selection as sites for cities were 
the good harbors, connected by navigable water with the 
open sea, and a good starting point, hence a good trading 
point, for the pioneers who penetrated the interior of the 
continent. The river valleys above the falls even where 
not navigable, furnish the best ways for first the trail, 
later the wagon road, and finally the railroad into the 



LAKES 137 

interior, and the early town thus grew into the modern 
city. 

Some of the more important cities whose sites were 
thus determined by the waterfalls of the type mentioned 
are Philadelphia, Baltimore, Washington, Richmond, 
Raleigh, Columbia, and Augusta. A line drawn through 
these cities is known as the Fall Line.* Trace out the 
line connecting these cities on a map of the United States. 

A great many prosperous manufacturing cities have 
grown up inland around waterfalls on the streams because 
of the valuable power obtainable for running the ma- 
chinery.- Let the class make a list of such cities. 

REFERENCES 

Lakes : 

Russell, Lakes of North America. Ginn & Co., 1895. 

Russell, Lake Lahontan, U. S. Geol. Surv. Mon., XI. 

Gilbert, Lake Bonneville, U. S. Geol. Surv., Mon., L 

Diller, Crater Lake, Oregon. Nat. Geog. Mag., Vol. 8, p. 33, 
and U. S. Topographic Atlas. 

Fenneman, Lakes of Southeast Wisconsin. Wis. Geol. Surv. 
Bull. 8. 

Murdock, The Great Salt Lake. Nat. Geog. Mag., Feb., 1903, 
p. 75. 
Swamps and Marshes: 

Shaler, 6th An. Report U. S. Geol. Surv. pp. 353-398. 

Shaler, 10th An. Report U. S. Geol. Surv. pp. 255-339. 
Niagara Falls, Grabau, Bull. 45 N. Y. State Museum, 1901. 
Niagara Falls, Gilbert, Nat. Geog. Mon., American Book Co. 



* It is thought that some of the falls on this line are due to faulting in the 
rocks. 



CHAPTER IV 
GLACIERS 

Switzerland has more visitors than any other country 
of equal area in the world, due chiefly to its beautiful 
scenery. One of the highest words of praise for the 
picturesqueness of any portion of our own country is to 
call it the "Switzerland of America." 

One of the most attractive features of Switzerland's 
beautiful landscapes is the occurrence of the fields of 
perpetual snow on the mountains sending down frozen 
rivers towards, often into, the green fields of the valleys. 
These streams of snow and ice, called glaciers, begin in a 
snow field and end in a river. 

117. Snow Fields.— The temperature of the atmos- 
phere decreases rapidly in ascending from the lower to 
the higher altitudes and latitudes. On the very high 
mountains in the tropics, on lower mountains in the tem- 
perate regions, and still lower elevations in the polar 
regions, most of the precipitation is in the form of snow. 
Where there is not sufficient warmth to melt all that falls, 
there is an accumulation from year to year which forms 
a snow field,— the perpetual snow of the snow-capped 
mountains. 

Small snow fields occur on a few of the higher peaks 
of the Rocky Mountains in the United States; larger ones 
on the high peaks of the Sierras; still larger ones further 
north along the Alaskan coast; and a much larger one 
in Greenland. In fact, the whole area of Greenland ex- 
cept a narrow strip along the coast, is covered with snow 

138 



GLACIERS 



139 



and ice. Small snow fields occur on the higher peaks in 
Mexico and Central America. There are snow fields, large 




Fia. 103. Aletsch glacier, the longest glacier in Switzerland. The dark 
line in the middle is the medial moraine, composed of rock fragments 
from the distant mountains. Notice the irregular surface caused by 
the numerous crevasses, and the deep depression on each side of the 
glacier. (Photograph furnished by Colgate University.) 



140 PHYSICAL GEOGRAPHY 

and small, on the higher mountains of the Alps, the 
Pyrenees, in Scandinavia, the Himalayas, the Andes, and 
in eastern Africa. The largest snow field in the world 
at the present time is that on the Antarctic continent. 

There is evidence that in the past there were larger 
snow fields than at present, one of which covered a large 
area in North America and another a large area in 
Europe. 

From Snow to Ice. If the snow continued to fall on 
the upland regions faster than it melted and there were no 
other escape for it, the final result would be to have all 
the water of the oceans, lakes, and rivers piled up around 
the poles and on the mountain tops. There is, however, 
another escape for the snow. It gradually changes into 
a granular mass composed of little pellets of ice, resem- 
bling coarse salt in appearance, known as the neve. It is 
similar to the last remnants of snow drifts in the spring. 
In the snow field under the heat of the sun and the pres- 
sure of the mass, this granular snow or neve is changed 
into hard blue ice, which gradually moves or flows away 
from the snow field. In the continental snow fields, like 
Antarctica, the ice flows out in a continous sheet around 
the margin, and in the smaller snow fields on the moun- 
tain peaks the streams of ice follow the deep valleys, lead- 
ing down the sides of the mountain. In some instances 
there is only one ice stream or glacier from a snow field. 
Sometimes several glaciers flow from the same field. 

There is no sharp separation between a glacier and the snow 
field which is its source. The ice that moves is properly called 
a glacier, but the bottom portion of the snow field consists of 
ice, most of it, probably all of it, in motion. 

Many Alpine snow fields lie in amphitheatre-like basins called 
cirques, which have been worn out of the solid rock by the ice. 
Fig. 104 shows several cirques formed in this way from which 
the glaciers have melted. 



UNIVERSITY 



OF 



Califo^ 



GLACIERS 



141 



Classes of Glaciers. (1) A continental glacier is a 
large field of snow and ice that covers a continent such as 
Antarctica or a large part of a continent such as the one 
that covered north central Europe or central North America. 




Fig. 104. Cirques or snow basins in the Rocky Mountains near Ouray, Co]. 
(July, 1906.) 1, a cirque. 2, ridge of moraine, composed of rock carried 
out of the basin. 3, another smaller cirque on the same ridge. 4, portion 
of another cirque only the top of which is visible. 



(2) The Alpine, or valley glacier, is the name given to the 
stream of ice that flows down the mountain valleys. Gla- 
ciers known as Piedmont are formed by the union of several 
valley glaciers which flow out on the same plain and unite 
into one. The Malaspina glacier in Alaska is an example. 
Cliff glaciers form in depressions on the mountain side, 
but do not extend down to the larger valleys. (See fig. 105.) 
118. Glacial Conditions.— Necessary conditions for- 
forming glaciers are (1) heavy snow fall, that is, a greater 



142 



PHYSICAL GEOGRAPHY 




Fig. 105. Work of a cliff glacier in the Rocky Mountains near Ouray, Col. 
The darker band across the Irght streak, just above the middle of the view 
is a terminal moraine of a cliff glacier that occupied the cirque above it. 
The talus cone below the moraine is composed of material carried by gravity 
down the mountain from the moraine. The loose material (2) above the 
moraine is mostly talus formed by frost action since the melting of the 
glacier. 

fall in the winter than will melt in the summer, and 
(2) a cool climate with changes of temperature. In the 
Himalaya Mountains, the glaciers on the south side, where 
it is warmer, descend several thousand feet nearer sea 
level than those on the north side, because of the greater 
quantity of snow precipitated on the south slope. Changes 
of temperature are needed both to cause precipitation 
of moisture and to change it to ice after it has fallen. 



GLACIERS . 143 

It is warmth that causes the moisture first to find its way 
into the atmosphere from the ocean and the moist land, 
and second to rise to the mountain top ; it is the cold that 
causes the precipitation in the form of snow, then the heat 
of the sun changes the snow first to neve and finally to ice 
and causes it to flow down the mountain. 

119. Movements of the Glacier.— It was a long time 
after the existence of glaciers was known, before it was 
understood that the ice really moved. Even at the pres- 
ent time there is some uncertainty as to just why and how 
it moves, but the fact that it does move is proven beyond 
question. After the study of this chapter, the pupil 
should enumerate all the points of evidence that the gla- 
ciers move. 

The rate of movement varies in different glaciers and 
even in the same glacier at different seasons. The move- 
ment is generally faster in a large glacier than in a small 
one under the same conditions. It is faster in a warm 
season than in a cold, faster in the middle than at the 
sides, faster at the top than on the bottom, faster on the 
outside of a curve than on the inside., faster on the 
steeper slopes. Why? 

One of the ways in which the rate of movement of 
different portions of a glacier was determined was to put 
a row of stakes across the ice in line with one on the rocks 
on each side. After a few months the straight line had 
the position shown in fig. 106. 

In 1821, two men were lost in a crevasse on the Bossons 
glacier on Mt. Blanc. Professor Forbes, who had been study- 
ing that glacier made the statement that the remains of these 
men would appear at the lower end of the glacier in 40 years, 
the length of time it would take the glacier to move from the 
crevasse to its lowest point, a distance of about one mile. In 
1861, when the mangled remains of these men, and some of 
the instruments carried by them appeared at the end of the 



144 



PHYSICAL GEOGRAPHY 



glacier, the forecast of Forbes 
veled at its accuracy. 



was recalled and people mar- 



120. Variation in Length of a Glacier.— The snow line 
is the elevation on the mountains above which the snow 
lies all the year, and below which it is all melted during 
the summer season. The glacier must originate above the 



B 



e • « e 



Fia. 106. Diagram indicating one of the ways in which movement of the ice 
in glaciers was demonstrated. A line of stakes was extended across a 
glacier continuous with those on the bordering mountain side. At suc- 
cessive intervals the stakes were observed in the positions shown. B 
shows the changes in a row of stakes on the vertical side of a glacier. 



snow line and as soon as it crosses that line it begins to 
waste from melting. The distance it descends below the 
snow line depends upon the size of the glacier, its rate of 
movement, and the climate in which it moves. It will 
melt nearer the snow line in the tropics and descend 
further below the snow line in the temperate and frigid 
climates. 

It is only in high latitudes that the glacier descends 
as low as sea level. In temperate and tropical regions it 
descends until the rate of melting just equals the rate of 
advance. At this point, which is the end or terminus of 



GLACIERS 



145 



the glacier, the ice is still moving, but it is melted as fast 
as it moves and the end of the glacier remains fixed as 
long as the conditions remain constant. 

One or more warm, dry seasons in which there is less snow- 
fall and more melting, will cause the end of the glacier to move 
back up the valley and establish a new point of equilibrium in 
accord with the new conditions. In the same way one or more 
cool, wet seasons in which there is unusually heavy snowfall 
and less rapid melting, will cause the end of the glacier to ad- 
vance down the valley a greater distance until conditions are 
again balanced. In this way the glacier serves as a good indi- 
cator of climatic variations. With one or two exceptions all 
the glaciers of the Alps are now retreating. In the early part 
of the last century they were advancing. 

121. Crevasses. — In the steeper parts of the chan- 




Fia. 107. Snowfield and crevasses, Alpine glacier. Some of the boulders on 
the surface fall to the bottom through the crevasses. Fresh snow some- 
times driffs over and closes the top of a crevasse thus forming a death 
trap for the unwary traveler who steps upon the snow and falls through 
into the depths of the glacier. (Colgate University.) 
10 



146 PHYSICAL GEOGRAPHY 

nel which would correspond to the rapids and waterfalls 
in the river, the glacier is very uneven, with many deep 
cracks or fissures called crevasses. Below the rapids these 
close up in part by regelation or refreezing and the ice 
stream passes on, as above the rapids. The crevasses are 
formed wherever there is a change in the angle of slope 
of the channel in which it is moving, and on the outside 
of the curve, where there is a bend in the valley. (Fig. 107.) 

122. Ice Tables and Pinnacles.— A large flat rock on 
the surface of the glacier protects the ice underneath from 
the sun's rays so that it does not melt as rapidly as the 
surrounding ice, the result being that the rock is finally 
left standing on a column of ice like a pyramid. When 
the surrounding ice is melted away several feet below the 
rock, the sun shines underneath the capstone, melting the 
ice on that side and the rock falls off, leaving a pinnacle 
or needle of ice. (Will the cap rocks fall on the north 
or south side of the pinnacle in the northern hemisphere?) 

The ice tables are more apt to occur near the lower end of 
the glacier. Why? They are more common in a dry, hot sum- 
mer than a cold one. Why? 

123. Other Surface Irregularities.— In a similar 
manner the medial moraine protects the ice from melting 
underneath it; so that it frequently forms a prominent 
ridge of rock fragments along the middle of the glacier. 
(See fig. 103.) 

The Alpine or valley glaciers, below the snow line, flow be- 
tween rock walls that are bare of snow in the summer season. 
The heat, absorbed and radiated on the glacier from the rocks 
where they receive the direct sunlight, causes the glacier to 
melt more rapidly on the side than in the middle. Hence, in 
crossing a glacier in such places one must ascend a hill of ice 
before reaching the middle of the glacier. (See lower end of 
Aletsch glacier fig. 103.) 



GLACIERS 147 

In the narrower canyons, where the glacier and the rocks 
bordering it do not receive the direct rays of the sun, the ice 
may be as high or higher at the sid6 than in the middle of 
the glacier. 

Small stones and thin patches of dust, instead of forming 
pinnacles like the large boulders, sink into the ice and make 
little holes called diist wells that are filled with water which 
freezes over at night and thaws during the day. 

124. Moraines.— As the glacier moves over the sur- 
face, it scrapes off large quantities of soil and even 
some fresh rock, especially where the underlying rock 
projects above the surface in sharp points or ledges. Be- 
sides the material pushed and shoved along in front of 
and underneath the ice there is a considerable quantity 
frozen in the lower portions of the ice. "Where the 
glacier flows along the base of a cliff, it receives all the 
rock fragments that fall from the cliff through the action 
of frost, gravity, and other weathering agencies. As the 
ice is advancing slowly all the time, the material is mov- 
ing forward, forming a band of rocky material along the 
side of the glacier below the cliff, known as the lateral 
moraine. Where two glaciers unite, the lateral moraines 
at the point of junction will unite in the midst of the 
combined glacier and form a medial or middle moraine. 
(See fig. 103). Where there are many tributaries, there 
may be many lines of medial moraines. 

In some instances the moraines are so numerous and 
so large as to entirely cover the surface of the ice, as is 
the case of the Unter Aar glacier in Switzerland. The 
material carried in the bottom and underneath the ice 
forms the ground moraine. Medial moraines may some- 
times be formed of material from the ground moraine or 
material underneath the glacier which is carried upward 
through the glacier by an upward current in the ice until 



148 PHYSICAL GEOGRAPHY 

it reaches the surface, forming a medial moraine or in- 
creasing the size of the one already formed. 

All the rock material carried by the ice, the medial, 
lateral and ground moraines, is dropped at the end of the 
glacier and there forms the terminal moraine. During 
the retreat of the glacier if the end remains stationary 
for a period of time, the material accumulates as in the 
first terminal moraine, from which it may be distinguished 
by calling it a recessional moraine or a terminal moraine 
of recession. (See fig. 105.) 

125. Special Topographic Forms of Glacial Deposit.— 




Fig. 108. Till or boulder clay on Fayette Street, Syracuse, N. Y. 
It consists of a mass of tough clay interspersed with numer- 
ous partially rounded, striated boulders. The entire mass 
was imbedded in the bottom of the ice or pushed along 
underneath the glacier. 

All of the material carried by the glacier is deposited 
somewhere, some of it at the end, some of it underneath 
the ice. That deposited at the end of the glacier, while 
it is stationary or nearly so, forms the terminal moraine 



GLACIERS 149 

which generally consists of irregular hills and ridges. 
The moraine in some places consists of a single ridge, 
while in others it occupies an area covered irregularly with 
hills of different sizes, sometimes inclosing basin-like de- 
pressions called kettle holes. When these are filled with 
water they are called kettle lakes and the whole area 
called the kettle moraine. Where the glacier retreats 
regularly, the moraine material is distributed somewhat 
uniformly over the area. 

Drift is the name given to all the material deposited 
by a glacier when it disappears by melting. 

Till, or boulder clay, is the unsorted material of the 
ground moraine consisting of clay, frequently blue clay, 
interspersed with more or less sand and boulders, the latter 
frequently striated and facetted by being rubbed against 
other pebbles or against the bed rock. (See fig. 120a.) 

A kame is a low hill of gravel and sand, partly but 
irregularly stratified, commonly elongated transverse to 




Fig. 109. Kame topography. Mendon, Monroe Co., N. Y. Karnes are 
composed of gravel and sand in irregular ridges, commonly formed in 
crevasses near the margin of the glacier. (H. L. Fairchild.) 

the direction of the ice movement. It is thought to have 
formed in cracks or crevices in the ice near the end or 



150 PHYSICAL GEOGRAPHY 

margin of the glacier, from materials washed in by sur- 
face streams and the waters from the melting ice and 
rains. 

An esker is a low winding ridge of sand and gravel 
generally elongated in the direction of the ice movement. 




Fig. 110. A small esker, near Jamesville, N. Y. Probably formed by the 
accumulation of gravel and sand in a stream channel underneath the 
ice. 

It may vary from a fraction of a mile to many miles 
in length and is probably formed by the accumula- 
tions in a sub-glacial stream which at the time would have 
sides and roof of ice. The esker is in general longer, 
lower, more winding and extended in a different direction 
from the kame. (See fig. 110.) 

A drumlin is a rounded egg-shaped hill composed of 
till or unstratified drift, commonly elongated in the direc- 
tion of ice movement and usually about 200 or 250 feet in 



GLACIERS 151 

height, sometimes lower but rarely higher than that above 
the base. Frequently there are pockets of sand and 
gravel in the surface of the drumlin, although the greater 
part of the outside consists of boulder clay. It is 




Pig. 111. Drumlin on Euclid Avenue, Sjrracuse, N. Y., looking east. 
The hill is composed of boulder clay deposited underneath the ice. 
In this case the north end is much steeper than the south end but in 
some drumlins the two ends are similar. 

probable that the central core of many drumlins is solid 
rock. Excavations either natural or artificial into the in- 
terior are so few in number that there is some uncertainty 
about drawing general conclusions regarding their inter- 
nal structure. Drumlins are formed underneath the 
glacier by the heaping up of the ground moraine. There 
are scores of drumlins along the line of the Erie Canal 
through central New York from east of Syracuse to west 



152 



PHYSICAL GEOGRAPHY 



of Rochester. They are abundant in eastern Massa- 
chusetts, in Wisconsin and Michigan. 

126. Corrading Work of Glaciers.— Glaciers are and 
have been important geologic agents both in grinding the 
rock surfaces and in transporting material. In corrading 
or grinding the surface rocks, the glacier acts like a 




Fig. 112. Glacial scratches (striae) on sandstone on tho 
Catskill Mountains at an elevation of 2,400 feet. (H. D. 
McGlashan.) 



coarse sandpaper pushed over the area. The continental 
glacier that covered the Northern United States must have 
been several thousand feet thick in many places and pushed 
its rough-shod surface against the rocks with tremendous 
force. 

Ice weighs over 50 pounds per cubic foot, hence a 
glacier 5,000 feet thick would press down with a force of 
more than 250,000 pounds on every square foot, but in 
many places the ice was 10,000 feet or more in thickness, 



GLACIERS 



153 



hence its corrading effect on the rock surface over which 
it moved must have been very great. 

The corrading action of the glacier is indicated by the 
scratching, grooving, and polishing of the rocks of all kinds over 
which it passed, and by the. vast quantities of ground-up fresh 
rock which it deposited along its course. Pulverized soil and 
mantle rock are prevailingly yellow, brown or red in color, but 




Fig. 113. Glacial grooves in volcanic rock on the side of 
Uncompaghre Canyon, near Ouray, Col. Extensive areas 
in this canyon are bare of any fragmental material and in 
many places show the abrading action of the glacier. 

ground-up fresh rock is blue. Besides the scratching and 
grooving of the bed rock, many of the loose boulders in the 
glacial deposit are striated, grooved, and partly rounded by 
being rubbed against other rocks, (Where possible, study the 
rock surface over which the ice has passed and carefully com- 
pare boulders from the glacial deposit with those from a 
stream channel, or from a sea or lake beach.) (Fig. 120a p. 161.) 
The grinding action was of course not uniform over the 
entire area. In a hilly region the tendency would be to wear 
away projections and make a smoother surface. The valley 
glaciers and the ice tongues of the continental glaciers extend- 



154 



PHYSICAL GEOGRAPHY 



ing in valleys, deepen and widen the valley, making a U-shaped 
glacial valley out of the V-shaped stream valley (fig. 114). How- 
ever, in passing across valleys transverse to the direction of 
movement, the tendency would be to deposit material in the 
bottom of the valley and wear away the top of the hill, thus 
making the surface more regular than it was before. We 
might expect also that in passing over a hill, a glacier would 




Fig. 114. A U-shaped glaciated valley near Ouray, Col. Much of the 
material from this valley was eroded by the glacier. Glacial cirque 
visible in background. There are three cirques at the head of the 
valley from which the united glaciers extended down the U-shaped 
valley in the foreground. 



corrade more on the stoss or thrust side of the hill (the side 
with which it first came in contact) than on the lee side where 
it might even deposit material. In places, basin-shaped depres- 
sions are scooped out in the rock which on the retreat of the 
glacier are filled with water and become lakes. (See the Silver- 
ton and Telluride quadrangles of the U. S. Topographic Atlas 
where scores of these cirque lakes are shown. Fig. 115 is a view 
in the northwest corner of the Silverton Quadrangle. See also 
Chap. III.) 

In many places in a flat region the glaciei moves over a bed 



GLACIERS 



155 



of sand or clay with almost no corrading action. In fact, in 
many places the corrasion is just as much on the hard rock as 
on the soft material and sometimes more. In this respect it is 
markedly different from the corrading action of a river, which 
always attacks the softest material first. 



fkiJ 








• ^« 





Fig. 115. Silver Lake Basin, near Ouray, Col. The lake in the foreground 
is formed behind a moraine. The low dark ridge in the middle of the 
picture is another moraine behind which is another lake similar to the 
one in the foreground. The whole basin including both lakes is a cirque. 



Over hard massive rocks the glacier frequently erodes 
the surface in small rounded domes which at a distance re- 
semble the backs of sheep and are called roches moutonnees 
(rock sheep). This type of glacial erosion is common on 
the crystalline rocks of the Canadian highlands and on the 
granite and volcanic rocks of the Rocky and Sierra Moun- 
tains. 

Huge pot holes, sometimes 20 feet or more in depth are 
formed in places underneath the glacier by the grinding of 
the boulders where they were whirled about by streams de- 



156 



PHYSICAL GEOGRAPHY 



scending through a crevasse. (See fig. 116.) Glacial pot 
holes are similar in some respects to those formed by 
streams on the rapids. (See sec. 72.) 




Fig. 116. Pot holes on grooved and striated rock surface in Glacier Garden, 
Lucerne, Switzerland. Larger pot holes are shown elsewhere in the same 
garden. They were formed by streams falling through the ice when the 
glacier covered the area, (J. C. Branner.) 

Hanging Valleys.— The glacier sometimes erodes the 
main valley much deeper than the tributary valleys and 
when the glaciers are melted from the area, the streams of 
water in the tributary valleys enter the main valley over 
a cliff, forming cataracts. Such tributaries are known as 
hanging valleys and are common in many glaciated areas. 

127. Glaciers as Transporting Agents.— The glaciers 
are probably more important transporting agents than 
corrading ones, since they not only carry all the material 
they wear off the rocks over which they pass, but large 
quantities that gain access to the glacier in other ways. 



GLACIERS 157 

It carries material frozen in its under surface and pushed 
underneath and in front, suhglacial material; material on 
the surface that falls from cliffs and mountains by which 




Fig. 117. Glacial Hanging Valley in Norway. The streams 
from the tributary valleys descend by a series of cascades 
to the floor of the main valley which was worn down by the 
glacier. 

it passes, and that swept on its surface by avalanches and 
otherwise— super glacial material; and that in the midst 
of the ice between the top and the bottom, englacial ma- 
terial. 

128. The Materials Carried by Glaciers.— The glac- 
iers carry all sizes of material from the very fine to the 
very coarse. The coarse and fine are sometimes sorted 
and separated by the waters associated with the melting 
ice, but frequently they are deposited in a heterogeneous 
mass. The bulk of the material in most glacial deposits 
over northern United States will be found on inspection 
to consist of rock materials that have come from an area, 
much of it within one or two miles of the place where it 



158 



PHYSICAL GEOGRAPHY 



is deposited. A part of the deposit, however, has been 
transported for a long distance, sometimes a hundred 
miles or more. Through central and southern New York, 




Fig. 118. A crystalline glacial boulder transported by the glacier from 
Canada to Syracuse, N. Y. Shown as it was being moved by man to 
the cemetery to serve as a monument. 

there are a great many boulders, large and small, that 
have been moved by the glacier from the Canadian high- 
lands north of Lake Ontario. One of these, known as the 
Grouse boulder, now in the cemetery at Syracuse, weighs 
about 75 tons, and was carried by the glacier from Canada 
to Central New York. 

The large boulders are mostly carried on or in the 
glacier and when the ice melts, the boulder is sometimes 
left on a very insecure foundation. In such positions 
they are called perched boulders or rocking stones. (Fig. 
119). Boulders of disintegration that sometimes resemble 



GLACIEES 



159 



glacial rocking stones may be distinguished by noting that 
they are the same kind of rock as that on which they are 
perched. Fig. 120 shows a boulder of disintegration in 
the Garden of the Gods. 




Fig. 119. Perched boulder or rocking stone deposited by the glacier near 
Greensboro, Vt. (C. H. Richardson.) 



The material transported by a glacier is deposited when the 
ice melts. As the greatest melting takes place at the lower end 
of the glacier, there the greatest deposits will be formed. In 
some places the material is spread somewhat evenly over the 
surface but frequently it is deposited in the form of drumlins, 
kames, eskers or the irregular mounds of a terminal moraine. 

The drift-covered surface of a region that has been covered 
by a continental glacier is quite different from the surface of the 
bed rock on which the drift material rests, and both the surface 
of the drift and the surface of the bed rock are different from 



160 



PHYSICAL GEOGRAPHY 



that of the same area before the glacier passed over it. Hence, 
the topography of a glaciated region is characteristic and dis- 
tinct from that of an unglaciated region. 




Fig. 120. Boulder of disintegration in the Garden of the Gods, Colorado. 
It is the remnant of a bed of sandstone; the surrounding portions 
have been carried away by rain and winds. It has not been trans- 
ported from another locality like the two preceding ones. (G. H. 
Ashley. ) 

129. Icebergs.— The ice composing the glacier is dis- 
posed of in two ways. In case of the large glaciers in 
high latitudes, the ice stream flows into the sea and moves 
out into the salt water until the end is broken off and 
floats away as a large cake of ice, called an iceberg. By 



GLACIERS 



161 




Fia. 120a. Boulders of different origin. 1 and 2 glacial boulders. 

and 5, boulders worn and polished by wind-blown Sand on the desert. 
6, boulder of disintegration. 7-11, boulders from a rocky beach. 

the action of ocean currents, icebergs slowly drift towards 
the equator, gradually melting as they move, until they 
finally disappear. The largest icebergs in the southern 




yiG. 121, Diagram illustrating how icebergs are formed where glaciers flow in- 
to the sea. The end of the glacier in the sea is raised and lowered by the 
tides and waves until it breaks off and floats away in an iceberg. 
11 



162 PHYSICAL GEOGRAPHY 

hemisphere come from the polar ice cap which covers the 
Antarctic continent. In the Northern hemisphere the 
largest ones come from the coast of Greenland. 

130. Melting of Glaciers.— Outside the polar regions 
glacial ice is disposed of by melting on the land before it 
reaches the sea. Melting goes on along the whole length 
of the glacier in the warm season, but is most active at 
the lower end where the air is denser and warmer. The 
water melting on the surface of the glacier forms streams 
which flow along the top until they come to open cracks 
or crevasses through which they drop to the bottom. 

In the Alpine or valley glaciers, the subglacial streams gen- 
erally unite under the ice and emerge from the end of the 
glacier in a single stream which frequently flows from an ice 
cave. 

In the case •of a continental glacier there will be many 
streams flowing from the margin of the glacier. Where the 




FiCJ. 122. View in une of the cross glacial channels, near 
Manlius, N. Y. The level floor of the channel, an eighth 
of a mile in width, is bordered by limestone cliflfs about 200 
feet high. The head of the channel is visible in the distance. 

land slopes away from the glacier, streams will run off through 
every depression or valley, and the vast quantities of rock ma- 



GLACIERS 163 

terial swept along by these streams is distributed down the 
valleys beyond the margin of the glacier and known as valley 
trains. In the absence of valleys, it will be spread out over the 
plain beyond the glacier, forming a glacial apron or an overwash 
plain. 

131. Glacial Channels.— Where the land slopes in the direc- 
tion from which the glacier is moving, there will be an accu- 
mulation of water in the depression at the end of the ice, form- 



:.:mf 





Fig. 123. View at the head of the channel shown in Fia. 122. 
The former vertical cliff has been partly eroded at the top 
and the falls are changing to rapids. 

ing a lake. The water from the melting ice here mingles with 
the water draining from the land, until it fills the depression 
and overflows at the lowest point. In central New York, 
where the glacier was moving south and the rivers flowing 
north, the ice formed a dam across all the north draining val- 
leys which then fllled with water up to the lowest point in 
the divide, when the water overflowed and often cut a deep 
channel through into the next valley. Where the water in one 
of these cross channels flowed over a ledge of hard rock on to a 
softer layer, waterfalls like Niagara were formed. Some of the 
streams near Syracuse were probably as large or even larger 
than the Niagara River. On the further retreat of the glacier 



164 PHYSICAL GEOGRAPHY 

the streams disappeared but the pools at the bottom of the 
waterfalls remained as lakes in the area mentioned. 

There are scores of these east and west cross-valleys formed 
in this way across the divides between the north-flowing streams 
in central New York. (For good examples of these cross chan- 
nels study the Syracuse, Tully and Skaneateles sheets of the 
contour map. See also maps, diagrams, and descriptions of 
these channels by Professor Fairchild in the 21st Annual Report 
of the State Geologist of New York.) (Figs. 122 and 123). 

132. Glaciers Compared with Rivers.— In some ways 
glaciers are like rivers; in others they are very much un- 
like them. Alpine or valley glaciers resemble rivers in 
flowing through valleys or elongated depressions in the 
surface and along the lowest part of the valley ; in having 
crooks and turns, and falls and rapids; in moving faster 
at the top than at the bottom, faster on the outside of a 
curve than on the inside; in moving more rapidly and 
having a rougher surface on the steeper portions of the 
channel, namely the falls and rapids, than on the level 
portions; in being fed by moisture precipitated from the 
atmosphere; in carrying this precipitated moisture to or 
towards the sea level; in carrying vast quantities of rock 
material from higher to lower levels. 

Glaciers differ from rivers in moving much more slow- 
ly—inches per day instead of miles. Both are fed by 
rains and snow, but rivers are fed chiefly by rain and 
groundwater, while glaciers are fed almost entirely by 
snow. The source of a glacier is always a snow field; of 
a river it is springs or a spring, a lake, or sometimes even 
a glacier. Glaciers carry more and coarser material on 
the surface than a river, which carries all its coarse ma- 
terial by rolling and pushing it along the bottom. 

Rivers may carry heavy loads down steep slopes, but 
they drop the greater part of the load on the first flat, 
while glaciers carry heavy burdens over flats and in many 



GLACIERS 165 

cases even up hill. The river carries most of its burden 
during the flood season, dropping much of it with the sub- 
sidence of the flood, while the glacier carries its burden 
steadily along until it drops it at the end of the ice or 
until it becomes lodged underneath the ice. Rivers wear 
away first the softer parts of the rock over which they 
flow, while glaciers wear away both hard and soft. Gla- 
ciers form moraines, rivers form flood plains and deltas. 
Rivers are frequently very important highways of com- 
merce, while glaciers are obstructions. In what other 
ways do glaciers and rivers resemble each other and differ 
from each other ? 

Glaciers form an important part in the freshwater circula- 
tion of the globe — a frozen portion of the circle that checks and 
retards the rate but fortunately does not fc:top it entirely. 

The North American Continental Glacier.— In the 

geological age immediately preceding the present, a large 
part of North America was covered with a great fleld of 
snow and ice. There were three centers of accumulation 
from which the ice moved out radially; one called the 
Labrador center was east of Hudson Bay, another the 
Keewatin center was directly west of Hudson Bay and 
another known as the Cordilleran was in Western Canada. 
From these three centers the ice spread out until it 
covered nearly all of Canada and a large part of northern 
United States. Many of the geographical features of this 
area are due directly or indirectly to the action of this 
now extinct glacier. It deepened and widened some of 
the valleys, it filled and destroyed others, it caused the 
shifting of many river channels. It changed the form of 
many of the existing hills and formed some additional 
ones. It scraped mantle rock from some places and de- 
posited it in others. 



166 



PHYSICAL GEOGRAPHY 



What are some of the other effects produced by the North 
American Glacier? Try to picture in your mind some of the 
results from the movement of such a great sea of ice into this 




Fig. 124. Map of North America — the ice age, showing the part covered by 
the ice and the three centers of accumulation from which the ice moved. 
(After Chamberlin. ) 



country now; its effect on the streams, lakes, hills, soil, vegeta- 
tion, animals and man; the conditions during the advance and 
those during the retreat or melting of the glacier. The condi- 



GLACIERS 167 

tions several thousands of years after the melting are those we 
have at the present time. 

To produce this great glacier there was of course a change 
in the climate, in fact, two changes, one an increase in cold to 
produce the ice and second a warmer change to cause its melt- 
ing and disappearance. The probable causes for these changes 
is a topic too large for discussion here. One cause was the 
elevation of the area to higher altitudes. Another probable 
cause was the variation of the amount of carbon dioxide in the 
atmosphere. (For good discussion see Geology by Chamberlin 
and Salisbury, page 424.) 

133. The Economic Effects of Glaciation.— After the 

study of glacial phenomena in the preceding pages the 
reader should draw his own inferences as to the effect of 
the glacier on the industries of man, on the area of the 
northern United States. The present soil is markedly 
different from that before the passage of the glacier. Has 
it been improved or not? In what respects? The topog- 
raphy is quite different. Is it better or worse for man^s 
use in agriculture? For transportation? The multi- 
tude of lakes were formed by the glacier. Are they an 
advantage or not? How? Most of the waterfalls are 
the result of glaciation. Are they a benefit or not ? How ? 
Enumerate other changes caused by the glacier, stating 
whether they have added to or detracted from the com- 
mercial value of the region. 

BEFEBENCES 

Russell, Glaciers of North America, Ginn & Co., 1897. 
Shaler and Davis, Glaciers, Houghton, Mifflin & Co., 1881. 
Wright, Ice Age in North America, D. Appleton & Co., 1890. 
Salisbury, Glacial Geology, Vol. V., Geol. Surv. N. X, 1902. 
Chamberlin, The Rock Scorings of the Great Ice Invasion, 

Tfrh An. Report U. S. Geol. Surv., p. 155. 
Upham, Glacial Lake Agassiz, Mon. 25, U. S. Geol. Surv. 
Geikie, The Great Ice Age, D. Appleton & Co., N. Y., 1895. 
Tarr, Phys. Geog. of N. Y. State, Macmillan Co., N. Y., 1902. 



168 PHYSICAL GEOGRAPHY 

Davis, The Sculpture of Mountains by Glaciers, Scottish 

Geog. Mag., Feb., 1906., p. 76. 
Fairchild, Glacial Lake Iroquois, N. Y. State Museum, 20th 

An. Report State Geol., 1900, Albany, N. Y. 
Fairchild, Drumlins of New York, Bull. Ill, N. Y. State 

Museum. 



CHAPTER V 

THE OCEAN 

A few centuries ago the ocean was an impassable bar- 
rier to man ; now it is the greatest and best of all his high- 
ways. Commercial products can be transported much 
cheaper across the ocean than the same distance across the 
continent, to say nothing of the greater ease and comfort 
to the traveller. Two of the principal factors in bringing 
about this change are the use of the mariner's compass 
and the improvement in steam navigation. 

134. Size of the Ocean.— The ocean is the irregular 
body of salt water surrounding and separating the con- 
tinents and containing, it is estimated, about 1,300 quad- 
rillion tons of water. It covers about 72 per cent of the 
earth's surface, or 143,259,000 square miles, of which 
7 per cent or 10,000,000 square miles lies on the con- 
tinental shelf. 

135. The continental shelf is the shallow margin 
of the ocean bordering the continents. It varies in width 
from a fraction of a mile to more than 100 miles. From 
the -outer or ocean margin of the shelf there is a steep 
slope or descent down to the ocean depths forming the 
sides of the basin. In other words, the ocean basins are 
full to overflowing and the overflow extends out over the 
border of the land areas forming an irregular belt of 
shallow water, the continental shelf, which corresponds 
in a way to an irregular rim of the submerged basin. 
Owing to elevations and depressions of the earth's crust 
the width of this shallow water zone varies greatly from 

169 



170 



PHYSICAL GEOGRAPHY 



time to time. A depression of the continent causes a 
further advance of the water on the land and the con- 
tinental shelf is wider. An elevation of the land area 
causes the recession of the shore line, the emergence of the 
shallow sea bottom and the continental shelf is narrower 



j-o^T^Conlinent 


o 
,^ CO Continental shelf 




^¥m%/. 


/"" / / ? 


V Ocean Basin 


y?7/777;fii^w7/7, 



Fig. 125. Vertical section across a continental shelf showing its relation to 
the continent and ocean basin. It varies from one or two to 100 miles 
or more in width. 



and the continent larger. Considerable portions of all of 
the continents have in ages past been covered by the sea 
and formed part of the continental shelf during their sub- 
mergence. (See fig. 125.) 

136. Mediterraneans.— Besides the open ocean there are 
several smaller divisions partially separated from it, the largest 
of which is the Mediterranean Sea, whose depth is nearly as 
great as that of the great oceans. It is almost entirely sepa- 
rated from the open sea, being connected with the Atlantic by 
the narrow strait of Gibraltar and with the Indian ocean by the 
Suez canal and the Red Sea. 

Other mediterranean seas are the Gulf of Mexico, the Car- 
ribbean, China, and Japan seas, the surface portions of which 
are not as nearly surrounded by land as the Mediterranean Sea, 
but their deep basins are surrounded by land. 

137. Composition of Sea Water.— Sea water contains 
much mineral matter in solution, the average being about 
3% per cent but it varies considerably in different parts 
of the ocean. The inflow of a great river like the Amazon 
or the Mississippi, or excessive evaporation in certain local- 
ities produces local variations in the percentage of salt. 
About three-fourths of the mineral matter held in solution 



THE OCEAN 171 

is sodium chloride or common salt. The remainder consists 
largely of magnesium, calcium, and potassium salts. There 
are minute quantities of other elements. 

Chemical composition of the salts of average sea- water : 

Sodium chloride 77.758% 

Magnesium chloride 10.878 

Magnesium sulfate 4.737 

Calcium sulfate 3.600 

Potassium sulfate 2,465 

Calcium carbonate 345 

Magnesium bromide 217 

100.000 
Besides the solid salts dissolved in the sea water, there is a 
large quantity of the gases of the atmosphere, which, like the 
salts, vary greatly in quantity in different parts of the oceaoi 
and in the same part at different times. Fishes and other animals 
of the sea obtain the oxygen necessary for life from the sea water. 
The carbonate of lime in the limestone beds on the land is 
dissolved by carbonic acid in the groundwater and carried into 
the sea in solution. When the corals and other animals se- 
crete the lime carbonate in their skeletons or shells, the car- 
bonic acid that was holding it in solution is set free and part of 
it at least goes back into the atmosphere. 

138. Circulation of Salts of the Ocean.— The water 
flowing into the sea carries salts in solution, while that 
which is evaporated is nearly pure water, which would 
apparently cause an accumulation of salt in the ocean. 
On the other hand, it is probable that much of the salts 
carried to the ocean are those that were formerly taken 
from the ocean. The great beds of rock salt in central 
New York and elsewhere were formerly in the sea. Nearly 
all the great beds of limestone over all the continents were 
deposited in the sea from materials taken from the sea 
water and are now being returned to the sea to be again 
extracted from the solution by animals and plants to form 
new beds of limestone over the sea bottom. Therefore, the 



172 



PHYSICAL GEOGRAPHY 



salts, like the water, circulate from the ocean to the conti- 
nents and back, and it is not possible from our present 
knowledge to say whether or not the sea water is becoming 
denser. 

Density of sea water varies with the temperature, the 
composition and the pressure. There is an increase in dens- 
ity with decrease in temperature to near the freezing point. 
It expands and becomes lighter as it freezes. The average 
density of the surface of the sea water at 60 degrees F. is 
about 106. There is a slight increase in density to the 

bottom due to the pres- 
sure of the overlying 
water. An increased per- 
centage of salts in solu- 
tion causes a correspond- 
ing increase in density. 

139. Sounding and 
Dredging.— Much defi- 
nite knowledge concern- 
ing the sea bottom and 
the deep portions of the 
sea has been obtained in 
the last half century by 
improved methods of 
sounding and dredging. 
The previous explorations 
had been in delineating 
the shore lines of the 
continents and islands. 
With the adoption of 
modern sounding lines 



w 



"zr 




Fig. 126. Sounding apparatus. The j j j xi u 

large ball, B, weighs several hundred and drcdgCS, a nCW Iield 

pounds and is mechanically detached q£ investigation WaS 
from the water bottle, W, when it 

strikes the bottom of the ocean. Opened, namely, the ocean 



THE OCEAN 



173 



bottom, the study of which has given rise to the new science, 
Oceanography. 

Soundings are made with fine steel wire (why not 
rope?) to which a sinker in the form of a heavy iron ball 
like a cannon ball is attached in such a way that it is re- 
leased when it 
strikes the bottom. 
Why are the sinkers 
left on the bottom 
of the ocean? Sam- 
ples of water are 
obtained from dif- 
ferent depths by at- 
taching to the wire 
at definite intervals 
brass tubes, called 
water bottles, so 
constructed that 
they remain open in 
descent but are au- 
tomatically closed 
as soon as lifting 
begins. 

The temperature 
of the ocean deeps 
is obtained by self- 
recording thermome- 
ters. These like the 
water bottles may be 
attached to the line 
at different places, 
so that a single 
sounding may give, 
besides the depth, 




Fig. 127. Two types of dredges used in collect- 
ing specimens of the mud and life forms 
from the bottom of the ocean. The bag 
is attached to a long wire from the ship, 
and is dragged along the bottom scooping up 
material. Some low forms of life are caught 
in the tangles below the dredge and brought 
to the surface in that way. 1, Chester rake 
dredge. 2, Blake dredge. (U. S. Fish Com.) 



174 PHYSICAL GEOGRAPHY 

samples of the water and the temperature at several differ- 
ent places between the surface and the bottom. Moreover, 
a sample of the bottom mud may at the same time be ob- 
tained by collecting that which sticks to the water bottle 
at the end of the wire. 

Specimens of the bottom sediment are generally ob- 
tained along with specimens of the life in trawls or 
dredges, consisting of strong nets having an iron rim and 
laden with weights. These nets when dragged along the 
sea bottom scoop up masses of the soft mud, ooze, and 
specimens of such forms of life as there exist. (Fig. 127.) 

There is another method of sounding by means of an instru- 
ment which records the pressure. One advantage of this 
method lies in the fact that it can be used without stopping the 
vessel, as it is independent of the length of line. 

140. The Deeps.— Scattered over the floor of the 
ocean basins are deep depressions— the so-called deeps or 
anti-plateaus y which extend below the ocean bottom to 
about the same extent that the plateaus rise above the 
general level of the continents. 

The deepest known point in the ocean is the Challenger 
deep, 31,600 feet, near our insular possession Guam. The 
Aldrich deep near New Zealand is 30,930; the deepest 
sounding in the Atlantic is near Porto Rico, 27,930. The 
Atlantic ocean is generally deeper near the sides, (15,000 
feet to 18,000 feet) than in the middle, (9,000 to 12,000 
feet). The elevated area of the mid-ocean bottom is 
called the Telegraph plateau and across it extends the 
several Atlantic cables from North America to Europe. 

The average depth of all the oceans is about 12,000 to 
15,000 feet, which is nearly six times the average height 
of the lands above the ocean level. It is estimated that if 
all the continents and islands were thrown in the sea the 
average depth would be nearly two and a half miles. 



THE OCEAN 175 

141. Temperatures of the Ocean.— The surface of the 
ocean is heated by the sun's rays, but these probably do 
not produce any perceptible effect below a few hundred 
feet. Since water, like air, grows lighter as it is heated, 
the surface-heated waters do not sink and hence do not 
reach the ocean bottom or any great depth in the ocean. 
The surface water in the equatorial region is heated to 
about 80 degrees F. At the poles it is frozen part of the 
year and near the freezing point most of the time. Since 
the colder water becomes heavier and sinks, all the water 
of the polar oceans is near 28 degrees F., the freezing 
point of salt water. As there is a slow creep of this water 
along the ocean bottom towards the equator, the deeper 
portions of the ocean, even in the equatorial regions, are 
very cold. Specimens of ooze and mud brought from the 
ocean bottom in the tropics show temperatures at or near 
the freezing point. 

The body of the ocean water has a rather uniform tem- 
perature. Even the surface waters change but little in 
comparison with the land temperatures, the daily change 
of the surface rarely exceeding two or three degrees and 
the yearly maximum range being fifteen degrees. 

Soundings of the Challenger in the Atlantic ocean, 3 J/2 
degrees south of the equator, show the following temper- 
atures : 

Surface 78 degrees Fahr. 

270 feet deep 68 

960 " " ' 50 

1920 " " 41 

9000 " " 36.5 

15200 " " 33 

The temperature of inland seas or mediterraneans is higher 
than that of the bordering ocean at corresponding depths be- 
cause they have no connection with the polar waters and do 
not have a temperature at the bottom lower than that at the 



176 



PHYSICAL GEOGRAPHY 



lowest place in the strait connecting them with the open sea or 
the coldest water formed in the winter season of the area. (See 
fig. 128.) 



ft 



CARIBBEAN SEA. 

70-80° 



AtL ANTIC OCEAN 

s^ 70-80° 



-39 H-- 
39M° 





Fig. 128. Diagram showing relation of temperatures in a mediter- 
ranean sea to corresponding depths in the open sea. The cold 
waters of the deep sea do not rise and hence do not pass the 
shallow water of the connecting strait. 

142. Waves. — Waves are formed on the sea or any 
body of water by the friction of the wind blowing across 
it, causing the surface water to move up and down, back 
and forth, each particle of water traversing an elliptical 
path. Generally the backward movement equals the for- 
ward and the water comes to rest where it started, except 
where the waves curl and break, when the top of the wave 
is driven forward. Where the wind continues for some 
time in the same direction, considerable quantities of 
water are driven forward and heaped up on the windward 
shores. (Fig. 129.) 




Fig. 129. Diagram illustrating the orbital movement of water particles in 
waves. The water rises in front of the advancing wave and sinks after 
the passing of the crest, each particle traversing a circular or elliptical 
path. A B level of water at rest. C C length of wave from crest to 
crest. D D' height of wave. 



THE OCEAN 



177 



Size of waves. The stronger the wind the larger the waves 
that are formed. The height of the wave measured from the 
bottom of the trough to the top of the crest, is sometimes 
thirty feet or more, rarely reaching a height of fifty feet in the 
open sea. The length of the wave varies from a few feet to 
1500 feet or more, much more in the earthquake waves, and the 
velocity varies from 20 to 60 miles an hour. The visible side of 
the advancing wave is the front, the opposite side the back of 
the wave. 

The size of the wave increases with the density, area, and 
depth of the water; hence the ocean waves are larger than 
those on lakes or rivers. 




Pit), lao. Breakers and surf on 'boulder beach. (M. S. Lovell.) 



143. Breakers.— As the waves of the open sea ap- 
proach the shore, where there is not sufficient depth of 
water to form the front of the advancing wave the top 
moves forward, breaks off, and falls as foam to be caught 
by the advancing wave and carried forward until it breaks 
again, in this way forming the so-called ''breakers" along 
the shore. It is these breakers that are so destructive to 

12 



178 PHYSICAL GEOGRAPHY 

boats and other property. Hence, when vessels cruising 
along the shore find a storm coming, if there is no good 
harbor near at hand, they sail for the open sea to avoid the 
destructive breakers of the shallow water. The white foam- 
ing waters produced by the breakers on the shore are called 
the surf. The turbulent waters, when not too violent, are at- 
tractive to the surf bathers. 

144. Undertow.— The undertow is a backward move- 
ment along the bottom from the shore towards the open 
sea. The water that is carried forward and heaped up on 
the shore by the breakers and surf cannot return sea-ward 
on the surface because of the incoming waves, so it flows 
back along the bottom, forming the undertow which so 
often proves dangerous to the surf bathers, who are 
caught by it and carried out into deep water and drowned. 
The fine material that is ground up by the waves on the 
beach is carried back into the deeper water by the under- 
tow and spread out in beds of gravel, sand, and clay which 
may later be elevated and form part of the stratified rocks 
of the continent. 

145. Earthquake Waves.— When an earthquake shock takes 
place beneath the bed of the sea, it sometimes causes the ele- 
vation of the surface of the water over a large area, which 
spreads out in long, low waves, having great velocity. As these 
waves approach the shore, they decrease in velocity but increase 
in height, piling up the water on the shore with great force, 
causing at times enormous destruction of life and property. 

During the disastrous earthquake that destroyed Lisbon in 
1755, the first shock caused the people who were not killed to 
leave their houses. Most of them assembled on the new marble 
quay, when the sea wave, 50 feet or more in height, swept in 
with great force, destroying nearly 60,000 people. 

The great volcanic eruption and accompanying earthquake 
shock at Krakatoa in 1883 produced sea waves that spread 
around the world. On the coasts near the eruption, waves 70 
feet or more in height rushed on the shore, destroying many vil- 



THE OCEAN 



179 



lages and thousands of people. So powerful were the waves 
that a large ocean vessel was swept a mile and a half inland 
and left there by the retreating wave. Earthquake waves are 
sometimes wrongly called tidal waves. 

146. Effects of the Waves.— (1) One of the most con- 
spicuous effects of the waves is the modification of the 
shore line produced by their erosive action. In this work 




Fig. 131. Wave eroded shore, Maryland. The indentations are worn by the 
waves assisted by gullies. The material is diatomaceous earth. (Maryland 
Geological Survey.) 



the common wind and storm waves are assisted by the 
tidal and the earthquake waves. They wear away rocks 
in some places and build up bars and reefs in others. The 
softer rocks are worn away first, forming bays and inlets 
between the harder rocks which form the headlands, or in 
some cases islands. (See Ciiapter VI). 

(2) The waves aerate the waters of the ocean by stir- 
ring them up and thus exposing larger surfaces to the 
action of the atmosphere; also by blowing over the crests 
of the waves, thus inclosing the air in the waters. This 



180 



PHYSICAL GEOGRAPHY 



action serves to oxidize the decaying: organic matter and 
thus purify the waters; it also furnishes oxygen for the 
animals living in the sea. 

(3) The. waves exercise enormous mechanical power, 
part of which is utilized by man to ring the bell and blow 




Fig. 132, Low tide in Bay of Fundy, near Gaspareaux River. See Fig. 
133. (Roland Hayward, 1903.) 

the whistle on the harbor buoys. This power is some- 
times used to pump water, or open flood gates. 

(4) The waves are frequently destructive to life and 
property. During violent storms they destroy sea walls, 
docks, lighthouses and other property on shore, and fre- 
quently overwhelm and destroy boats. The destructive 
effect of the waves on boats in the open sea is materially 
lessened and often the vessel is saved by spreading a little 
oil on the water. The disastrous effects of the waves are 
produced by the breaking of the wave, when the top curls 



THE OCEAN 181 

over and falls upon the boat. A little oil on the water 
spreads rapidly even in the face of the wind, and decreasejj 
the friction enough to permit the crest of the wave to 
settle back quietly without breaking. Small boats can 




Fig. 133. High tide in Bay of Fundy, same point as FiG. 132. (Roland 
Hayward, 1903.) 

safely ride the largest waves as long as the waves do not 
break and fall into the boat. 

147. Tides and Tidal Waves.— At all points on the 
shore of the ocean the water rises and falls twice each da^^ 
It rises steadily for about six hours until it reaches its 
highest level, high tide, and then subsides for about six 
hours, until it reaches its lowest level, low tide, when it 
again rises. The period of rising is not always uniform 
with the period of falling, but the average of the sum of 
the two is equal to 12 hours and 26 minutes. Figs. 132 and 
133.) 



182 PHYSICAL GEOGRAPHY 

Twice each month the tides reach a maximum height, 
the spring tide, and twice they reach the minimum height, 
the neap tide. The incoming tide is called the flood tide, 
the outgoing the ebb tide. Slack water is the interval be- 
tween the two. 

In shallow harbors the hour of departure of ocean steamers 
is usually determined by the time of high tide, as they can then 
float with safety over the bars and shallow places which they 
could not pass at low tide. Finding the time when high tide will 
occur at any place is called "establishing the port." 

On the open sea the rise and fall of the tide is not percep- 
tible, so low and broad is the wave. In bays and estuaries where 
the tidal wave is confined and restricted, it frequently rises to 
great heights. In the Bay of Fundy on the coast of Nova Scotia 




Fig. 134. Tidal flats. — Low tide in Basin of Mines, N. S. The area is 
covered with water at time of high tide. (S. R. Stoddard.) 

the tide rises to a height of 50 feet or more. Similar high tides 
occur in the Bristol channel. In both places the tide is not con- 
spicuously high at the comparatively wide mouth of the bay, but 
as the low, long wave advances up the ever-narrowing channel 
the waters begin to pile up until they reach a maximum at or 
near the head of the bay. (Figs. 132 and 133). 

148. Tidal Wave in Rivers.— In certain places the 



THE OCEAN 183 

tidal wave meets opposition in the current of a river and 
at times the waters rise into a high wave commonly known 
as the hore or eagre which rushes up the river, often with 
high velocity, causing great destruction along the banks 
and at times to shipping in the river. On the Amazon 
.River this tidal wave, known as the pororoca, extends for 
several hundred miles up the river with great destruction 
to the bordering forests. Similar waves often prove very 
destructive to shipping on the Hoang Ho (River) in China 
and the Seine River in France. 

Tidal race. In Long Island Sound a low tide from the east 
meets a high tide from the west at Hell Gate and six hours later 
the conditions are reversed. At both times the water rushing 
through the narrow channel with great velocity proved very 
destructive to shipping until the channel was widened by blast- 
ing away the rocks. Such a current is called a tidal race. 

A somewhat similar but more complex meeting of the tides 
in the North Sea forms the dreaded Maelstrom oft the coast of 
Norway. 

149. Cause of the Tides.— The close relation existing 
between the time of the tides and the time of the succes- 
sive crossings of the meridian by the moon was known 
for a long time before it was suggested that gravitative 
attraction was probably the cause of the tides. The at- 
traction of the moon causes the heaping up of the waters 
on the side towards the moon, because they are nearer and 
because the waters respond more readily to the gravitative 
pull than do the more rigid rocks. For the same reason 
the waters would rise on the side opposite the moon. The 
heaping up of the waters on opposite sides of the globe 
causes a lowering of the waters or low tide at the inter- 
mediate points. (See fig. 135.) 

The sun also causes a tide, but so much smaller than 
that caused by the moon that it is rarely noted except 



184 



PHYSICAL GEOGRAPHY 



when it coincides with that of the moon or is directly op- 
posed to it. 

Spring and neap tides. The sun tide coincides with the moon 
tide when the sun and moon are in line with the earth, either 



o 



— 



6 



a. 



B 




Fig. 135. Diagram showing position of the sun and moon during spring and 
neap tides. A spring tide, the result is the same at full moon when the 
moon is on the oposite side from the sun. B, neap tide. First quarter; 
the result is the same at the third quarter of the moon, 

on the same side, as at new moon, or on opposite sides, as at 
full moon. The tides are then equal to the sum of the two, the 
greatest for the month, and are called spring tides. 

During the moon's quarter the sun and moon are at right 
angles to each other from any point on the earth, when the tide 
equals the difference between the two and hence is the lowest 
for the month, the neap tides, which come at the first and the 
third quarters. 

Owing to the inertia of the water, it takes some time for the 
full effect of the moon's attraction to manifest itself, so that 
high tide Is not directly underneath the moon, but some distance, 
at times some hours behind it. This is the lag of the tides. 
The regularity of the movement of the wave is disturbed very 
much by the continental and insular land masses, so that in. 
many places the tidal movements become quite comple«. 



THE OCEAN 185 

150. Ocean Currents.— The very slow, almost imper- 
ceptible movements of the ocean waters are called creep; 
the faster but still very slow movements are called drift; 
the faster, more conspicuous movements are called cur- 
rents, and the more rapid currents are called streams. In 
^the waves the movement of the water is mainly up and 
down, but in the strong wind waves where they break at 
the top, forming the "white caps," there is a forward 
motion at the top of the wave which is blown over and 
driven forward by the wind. A continuation of this 
movement, as in the belt of constant winds, would pro- 
duce a surface current. 

151. Causes of Ocean Currents.— The causes given for 
ocean currents are winds, differences in temperature and 
pressure, and the rotation of the earth. There is some 
difference of opinion concerning the relative importance 
of these. The movement of the winds is probably the most 
important cause of all. The difference in temperature be- 
tween the warm tropical waters and the cold polar waters 
would cause convectional movements. The ultimate cause 
in both cases is difference in temperature, but in the first 
case it produces movements of the atmosphere, which in 
turn cause movements of the water. The rotation of the 
earth probably produces movement of the ocean waters to 
some extent. The direction of the movements is influenced 
by the rotation of the earth and by the outline of the con- 
tinents. 

152. Currents in the Atlantic— In the Atlantic Ocean 
there is a current westward, in the equatorial regions, to 
the South American coast, where part of it is deflected 
into the South Atlantic and part north. The north 
branch divides at the West Indies, ' part of it passing 
through the Carribbean Sea and the Gulf of Mexico, 
whence it emerges as the Gulf Stream. After joining the 



186 



PHYSICAL GEOGRAPHY 



other portion of the equatorial current east of Florida, it 
continues northeast across the ocean as the Atlantic drift, 
dividing again west of Europe where a portion of it con- 
tinues northeast until it is lost in the Arctic ocean. The 
other part of the Atlantic drift turns southward along 
the coast of Spain and Portugal and northwest Africa, 








^AUs^ALiA^ ^ ^ ^ ^SOUTH PACIFIcy^ M " Fl/^' A T L .» N T i c ' 'iK- yy C ' o o V 



\- r""^.-*'l.N--''T,,--'A.';T.R'''c ,-T' l-'-fc' E ..D-D- Y" 



Cbait «t U>e Ocean Cturents. 

Fig. 136. Map of ocean currents. The solid lines are warm currents; the 
dotted lines are cold currents. Sargasso seas in each of the eddies. 



until it finally joins the equatorial current to be again 
turned westward across the Atlantic, thus completing the 
circuit of the North Atlantic Ocean. Trace out in a 
similar way from the map the warm currents and then 
the cold ones of the other oceans. (Fig. 136.) 

153. Sargasso Seas.— The central portion of the North At- 
lantic, the part surrounded by the current just described, hag 
no continued movement but corresponds to the central portion 
of a great eddy. It is called the Sargasso sea from the abun- 
dance of the seaweed Sargassum which, because of the numerous 
air sacs along the stem, floats on the surface of the ocean. 
Under the combined action of the eddying waters and the shift- 
ing winds, this floating weed accumulates in places in such dense 



THE OCEAN 187 

masses as to seriously retard the progress of ships. It was part 
of the Sargasso sea that Columbus encountered in his first voy- 
age, where his sailors became frightened, thinking they might 
never escape. Where are the other sargasso seas? 

154. Drift of Cold Waters.— Slow movements of the 
ocean water are called drift or creep. There is an ex- 
tensive creep of the cold polar waters toward the equato- 
rial regions which appears as surface movements only in 
high latitudes, and only locally does the movement form 
true currents. When the south-moving cold polar waters 
meet the north-moving warm currents or drift, they sink 
underneath the warm waters and continue to creep 
equatorward along the ocean bottom. Ferrel's law (see 
index) about the movements of the atmosphere applies 
equally well to the ocean currents. From the map show 
or explain the relation. 

155. Effects of Ocean Currents.— (1) The movement 
into the higher latitudes of large bodies of warm water, 
like the Gulf Stream in the Atlantic and the Japan cur- 
rent in the Pacific, carries with it tropical heat which 
tempers the climate in a marked degree. Likewise the 
polar currents and the drift bring the cold of the polar 
regions into the lower latitudes and cool the climate. 

(2) Ocean currents affect navigation by hastening or 
retarding the speed of vessels, depending upon whether 
they are going with or against the current. Sometimes 
they cause vessels to drift from their courses on to dan- 
gerous coasts, when the current is toward the shore along 
which the vessel is sailing. Because of these shifting 
movements, often imperceptible, the navigator must use 
extra precautions in planning his course and in getting 
his location by astronomical observations. Nansen at- 
tempted to reach the north pole by getting in the waters 
drifting northward and permitting his vessel to be frozen 



188 PHYSICAL GEOGRAPHY 

in the ice and carried along with the drift. The lack of 
definite knowledge concerning the polar drift resulted in 
failure to reach the pole. 

(3) Ocean currents distribute plant life. The islands 
of the sea receive seeds which have drifted from distant 
lands. Many of the verdure-clad islands that would 
otherwise have remained barren land have received their 
vegetation in this way. In a similar manner, both lower 
and higher forms of animal life have been carried long 
distances by the ocean currents and thus introduced into 
other lands. 

(4) The transference of water from one part of the 
ocean to another by the currents prevents stagnation and 
makes life possible by carrying oxygen and food to the 
different organisms. The currents are instrumental in 
causing dense fogs by bringing cold and warm water and 
hence cold and warm air together in large quantities. 

Explain the cause of the dense fogs that occur so frequently 
on the fishing grounds of the banks near Newfoundland. Since 
many of the trans-Atlantic steamers pass over these banks the 
danger of collision with the fishing vessels is greatly increased 
by the heavy fogs. Why is London noted for its fogs? Why 
should there be more fog at San Francisco than at Denver? 

That the colder, heavier polar waters creep along the ocean 
bottom towards the equator is shown by the low temperature 
of the water at great depths in tropical regions. This move- 
ment is probably universal but very slow in all oceans. The in- 
creeping cold waters replace the warm water carried from the 
warmer regions by evaporation, and thus complete a general 
circulation of all the oceanic waters. 

THE OCEAN FLOOR 

156. Topography of the Ocean Bottom.— The broad- 
er general features of the ocean bottom are not greatly 
different from those of the land areas, but the details are 
decidedly different. There are plains, plateaus, and 



THE OCEAN 189 

mountains, but there is an almost total lack of valleys and 
hills that mark the continental land areas. Hence if the 
sea bottom were exposed to view one would be impressed 
by the striking monotony of the scenery, the absence of 
the many varied forms sculptured on the land by rainfall, 
winds, and streams. 

In places on the continental shelf where portions of the land 
area have been recently submerged, the buried hills and valleys 
are not entirely obliterated. 

157. Materials on the Sea Bottom.— The materials 
on the sea bottom are quite varied in different places, but 
may be divided into those on the continental shelf and 
those over the deep sea basins. The first would include 
those deposited in water less than 600 feet deep and would 
consist of gravel, sand, and mud; coral, and other organic 
deposits, the materials of which are derived mainly from 
the lands. 

The materials eroded from the land by the rivers and 
from the beach by the ocean waves are carried out and 
spread over the sea bottom by the river and shore currents, 
by the undertow and by winds which carry it as dust 
through the atmosphere. The sand and mud are carried 
in suspension and dragged along the bottom, while the 
lime carbonate for the limestones is carried out in solu- 
tion. 

The mechanical sediments are generally coarser and 
thicker within a few miles of the shore, thinning out in 
the deeper waters. The calcareous deposits are formed 
in the clearer waters which are comparatively free from 
sediments. 

158. Deep Sea Deposits.— The deeper portions of 
the sea— the deep basins outside the continental shelf- 
are covered with organic oozes and fine muds. Some of 
the oozes are calcareous or limy, and some silicious, that 



190 



PHYSICAL GEOGRAPHY 



is, composed of silica. The most common of the calcar- 
eous oozes, named from the prevailing forms of organic 
remains, are the glohigerina ooze and the pteropod ooze; 
the siliceous ones are the radiolarian ooze and the diatom 
ooze. The first three are minute animal forms; diatoms 
are microscopic plants of varied and beautiful forms 
which live in both salt and fresh water. (See sec. 105, 
chapter III.) 




Fig. 137. Some of the deep sea ooze highly magnified. The minute animals 
live at or near the surface of the sea biit their remains sink to the bottom 
where it forms ooze. A — foraminiferal ooze magnified 50 diameters 
from depth of 11,000 feet. B — raliolarian ooze magnified 100 diameters 
from depth of 26,850 (one of the deeps.) 

The microscopic plants and animals which form the 
different oozes, live on or near the surface of the sea and 
as they die, their remains sink to the bottom and accumu- 
late as the soft ooze. They live at the surface in shallow 
water as well as in mid-ocean but there is so much other 
miaterial on the bottom of the shallow seas that the re- 
mains of the microscopic organisms are generally ob- 
scured, while in the deep sea they form the bulk of the 
material on the bottom. 

In many places around the border of the oceanic basins 
there are extensive areas of fine muds, named from their 



THE OCEAN 191 

color blue, red, and green. In the deepest portions of 
the sea basins, known as the deeps, the bottom is covered 
with a red clay, the origin of which is uncertain, but it is 
probably formed in part at least of volcanic and meteoric 
dust. Glauconite or green sand covers the sea bottom 
in some places. 

The greater part of the continental areas is covered with 
rocks that were formed in the sea. Even the greatest mountain 
ranges and the extensive plateau areas are largely composed of 
the materials of former sea bottoms. It is quite probable that 
portions of the present sea bottom over the continental shelf may 
in the future become land areas not greatly different from the 
present lands. 

Most of the rock over the continents consists of sand- 
stones, shales and limestones, the off-shore shallow water 
deposits; yet there are representatives of the oozes in the 
chalk beds of England, Prance, and portions of the United 
States. There are diatomaceous deposits of great extent 
in California, and smaller deposits at Richmond, Virginia, 
and elsewhere. 

159. Stability of Sea Basins and Continents.— Despite 'the 
fact that the continents are for the most part covered with sea 
bottom deposits, there is good reason for thinking that there is 
little or no change from sea basin to land area and vice versa. 
The interchange has been between the continents and the con- 
tinental shelf. At the present time the area of the continental 
shelf is about 10,000,000 square miles. From time to time por- 
tions of it are elevated above the sea level and added to the con- 
tinents and portions of the continents are depressed and added 
to the sea area. Diastrophic movements (compare first part of 
Chapter VIII) of this kind have produced many changes in the 
land and sea areas during the past geological ages. Through them 
all the continents have probably been growing larger and the 
continental shelf smaller. The ocean basins have doubtless 
been growing deeper but not larger in area. 

160. Life in the Ocean.— The life in the ocean is quite 
varied and in places very prolific. The varying physical 



192 PHYSICAL GEOGEAPHY 

conditions produce three rather distinct life regions: (1) 
the continental shelf or shallow water area, (2) the bot- 
tom of the deep sea basins, (3) the pelagic life or that on 
the surface of the open sea. 

Life on the shore and in the shallow seas. The life of 




Pig. 138. Two of the larger animals of the open ocean. The upper one is a 
grampus or whale which has been stranded on the beach. The other 
is a bottle-nosed dolphin. (U. S. Fish Com.) 

the shallow seas includes the greater part of the more 
familiar forms such as the fishes, molluscs, crabs, lobsters, 
corals, sea urchins, star fishes, porpoises, and seals, all in 
great number and variety. In places there is also prolific 
vegetable life which is necessary for the support of the 
animal life. The shallower portions of the ocean, known 



THE OCEAN 



193 




Fia. 139. Giant squid, Port Otway, W. Patagonia. It frequents the 
shallow water near shore in cold climates. It is a large and voracious 
animal belonging to the same class as the devil fish. (U. S. Fish Com.) 

13 



194 PHYSICAL GEOGRAPHY 

as banks, such as the Grand Banks off the coast of New- 
foundland, are frequented by vessels from distant lands 
for the cod and other fishes which swarm here in great 
numbers. The coral reefs, which grow only in shallow 
water, teem with multitudes of living forms. Indeed, 
there are few places where life is more prolific than on a 
coral reef. The littoral or shore life includes the eel grass, 
marsh grass, mangrove, and other plants, besides the great 
variety of animal life. (Figs. 138 and 139.) 

Deep sea life. The life on the deep ocean bottom is 
quite meagre, consisting of a few strange and fantastic 
forms. The conditions are unfavorable for abundance of 
life, as it is everywhere dark, cold,— almost at the freezing- 
point— and there is enormous pressure from the great 
depth of water. 

Despite the dreary, monotonous, and undesirable con- 
ditions of deep sea bottom, there is a growth, scanty it is 
true, of living forms over a considerable portion of it. 
There is probably no vegetation, as that requires some sun- 
light, and hence the animals of the sea bottom are de- 
pendent for their food supply on the remains of the sur- 
face forms which sink to the bottom. 

Many of the deep sea forms, as they are brought up In 
dredges, perish as soon as they reacli the surface because of 
the great change in pressure. At a depth of 20,000 feet there is 
a pressure from the overlying water of over 600 tons per 
square foot. The animals of the deep sea resist this pressure 
in the same way that we resist the pressure of the atmosphere, 
namely, by having a corresponding pressure on the inside. When 
they are somewhat suddenly released from the great external 
pressure, before there is a proper adjustment of the internal 
pressure, the result is generally disastrous. 

Pelagic life. The life on and near the surface of the 
open sea— the pelagic life— is nearly everywhere abundant, 
but especially so in the tropical regions. There is a great 



THE OCEAN 195 

variety as well as quantity of forms, varying in size from 
the multitudes of microscopic plants and animals (see sec. 
158) to the huge whales which feed upon them. 

The floating vegetation of the sargasso seas attract many 
forms of animal life which make it their feeding ground, 
and they in turn form food for carnivorous forms which 
are thus attracted. 

The different forms of pelagic life, including the floating 
vegetation, the microscopic animals, whales, fishes, and other 
free swimming forms, are common also in the surface waters of 
the shallow water areas of the continental shelf. The range of 
the different species over the surface of the ocean is limited by 
the changes in temperature and not by the depth of the water. 

The great part of the pelagic life lies on or close to the sur- 
face of the ocean, between which and the sparsely inhabited sea 
bottom is the great bulk of the oceanic waters — dark, cold, 
dreary, monotonous zones — great, watery desert areas, almost 
barren of life. 

161. Economic Features of the Ocean.— "Old Ocean's 
gray and melancholy waste," like many other poetical ex- 
pressions, is very misleading and untrue if we attempt to 
apply it literally. It is the greatest and by far the best 
of all our highways, which, besides being free, extends to 
nearly every nation and serves to unite the civilized coun- 
tries into a great commercial family. 

The ocean makes the land habitable by furnishing 
moisture and tempering the climate. It carries the 
warmth of the tropical sun to the temperate and polar 
regions and in turn transports the cold of the poles to- 
wards the equator. It is the chief factor in the circula- 
tion of moisture through the atmosphere. 

An important part of the food supply of the world 
comes from the ocean. Probably most important of all 
are the fishes of many kinds. Make a list of the names of 
all the fishes that you know are taken from the ocean for 



196 PHYSICAL GEOGRAPHY 

food. Besides the fishes there are oysters, clams, lobsters, 
crabs, shrimps, walruses, polar bears, whales, porpoises, 
and seals. Other important products are pearls, coral, 
sponges, shells, salt, and seaweed. In some countries sea- 
weed is used extensively for food. What other uses has 
it? It is estimated that the annual value of the food 
products taken from the sea is not less than $500,000,000. 



REFERENCES 

Pillsbury, The Gulf Stream, the Annual Report, U. S. Coast 
Survey, 1890. 

Page, Ocean Currents, Monthly Weather Review, August, 
1902, p. 397. 

Davis, Winds and Ocean Currents, Journal of School Geog- 
raphy, Vol. II, 1898, p. 16, 

Everman, Strange Fishes of the Deep Sea, The World of To- 
Day, June and September, 1902. 

Goode, Deep Sea Fishes. 

Thomson, The Depths of the Sea, and the Voyage of the 
Challenger; Macmilla^n Company. 

Sigsbee, Deep Sea Sounding and Dredging, Washington, D. C, 
1880. 

Tanner, Deep Sea Exploration, U. S. Fish Commission, Wash- 
ington, D. C. 

Flint, Oceanography of the Pacific, Bull. No. 55, U. S. Natl. 
Museum. 

Agassiz, Three Cruises of the Blake, 2 vols., Houghton, 
Mifflin & Company, Boston, 1888. 

Maury, Physical Geography of the Sea. 

Littlehales, Marine Hydrographic Survey of the Coasts of 
the World, 8th Rept. Int. Geog. Cong. Washington, 
1904, p. 576. 



CHAPTER VI 
SHORE LINES 

162. The shore line is the line where land and sea 
areas meet ; the line above and below which there is an 
abrupt change in the living forms and sometimes a very 
sudden change in the topographic forms. It is on and 
near the shore where great numbers of plant and animal 
remains are buried and preserved in the shore sediments, 
hence furnishing a key to the life of the geological period. 
Thus by studying the fossil forms in the shore deposits of 
the past, one gains a knowledge of the life of previous 
ages. Animals and plants living during one geological 
period are different from those of the preceding and fol- 
lowing periods ; hence the fossil remains in the ancient 
shore deposits indicate the geologic age in which the de- 
posits were formed. (See fig. 140.) 

163. Topography.— The shore line has a topography 
peculiarly its own. The bars, beaches, projections, in- 
dentations, and deposits are in many respects different 
from anything inland or on the sea bottom. A knowl- 
edge of shore forms and shore features aids greatly in the 
study of the geography of the past, the distribution of 
land and sea areas in the past ages. 

Besides the lessons taught by the shore features and de- 
posits, there is a wonderful inspiration in standing on the shore 
and contemplating the vastness of the ocean and the ceaseless 
beat and roll of the waves. It appeals to man in his con- 
templative moods in much the same degree, but in a different 
manner from that of the grandeur of the mountains, or the soli- 
tude of the desert. 

197 



198 



PHYSICAL GEOGRAPHY 



Like the river, the mountain, the plain, and other natural 
features, the shore line appears to follow a more or less regular 
cycle of change and have a life history of its own. But because 
it is more sensitive to slight changes of elevation or depression 
and probably more subject to such changes, the shore cycle is 
liable to be interrupted or broken so frequently that its exist- 




iiu. 140. Bodkin Point, Md. A buried cypress forest ao-w being uncovered 
by the waves. Remains of both the ancient forest and the living one are 
being buried in the sands of the present beach to tell the story in future 
ages. (Maryland Geological Survey.) 



ence is commonly overlooked and many persons thus fail to read 
the past history of a shore by studying its present features. Yet 
the phenomena on any shore line, if properly interpreted, indi- 
cate many of the changes it has undergone and will undergo. 

164. Shore Erosion. — The erosion on the shore line 
is done mostly by the waves, which erode the rocks much 
as the rain and the streams do the upland. The corrad- 
ing action of the waves is aided by the other weathering 



SHORE LINES 199 

agencies, such as the frost, wind, chemical action, plants, 
and animals. The tides assist the wave action by lifting 
and lowering them to new points of attack. 




Fig. 141. Pacific Coast on the 17-mile drive at Monterey, Cal. Showing 
breakers and the effect of the waves on hard rocks. Why are the rocks 
on the beach more rounded than those out in the water? (Detroit Pub. 
Co.) 

The active work of the waves is done mostly during a 
storm, or by the irregular heavy earthquake (so-called 
tidal) waves that occasionally deluge a coast. The storm 
waves on the coast correspond in their corrading effect to 
the freshet in the river. 

165. Effect of Storm Waves.— The work of the storm 
waves is confined largely to a vertical range from about 
50 feet below tide to 100 feet or so above tide. On this 
comparatively narrow vertical belt their force is frequent- 



200 PHYSICAL GEOGRAPHY 

ly terrific and their work is accomplished in several ways: 

(1) The impact of the boulders, shingle, and sand, that 
are picked up by the waves and hurled against the rocks 
with great force, loosens more material from the cliff and 
breaks and grinds up that already loosened. On the 
Bahama Islands, blocfe weighing twenty tons have been 
hurled by the waves 125 feet from the shore and 25 feet 
above high water. 

(2) The boulders, gravel and sand torn from the cliff 
are rolled up and down the sloping beach, being thus worn 
smaller, while the finer material is swept back into deeper 
water, 

(3) The spray acts both mechanically and chemically. 
On the coast of Scotland, light-house windows have been 
broken at the height of 300 feet. The spray dashing 
against the rocks, .high above the reach of the waves, as- 
sists in the chemical disintegration of the rock constituents. 

(4) The hydrostatic pressure of the water, and the 
compression* and expansion of the air driven by the waves 
into the. crevices and caves along the shore, are agents of 
disintegration. 

166. Tidal Waves.— True tidal waves are not very powerful 
eroding agents on the open shore, but in bays, estuaries, and 
river channels they are often quite active. The hore or pororoca 
of the Amazon, the mascaret of the Seine, and similar waveg 
in other rivers, are great tidal waves which sweep with high 
velocity up the river channel, destroying and tearing away the 
material along the banks, proving destructive to boats on the 
river as well as to property on the shore. The so-called tidal 
wave that destroyed so much property and flooded such ex- 
tensive land areas at Galveston in 1902 was produced by the 
hurricane which it accompanied and was in no way related to 
the tide. 

167. Earthquake Waves.— Sometimes a coast or a 
portion of a coast is visited by a large and very destruc- 



SHORE LINES 



201 



tive wave frequently but wrongly called a tidal wave, as 
it in nearly all eases accompanies an earthquake or vol- 
canic disturbance. Such waves are not destructive on 
the open sea, where they pass as low waves of great length 
and often unperceived; but as they approach the 'shore 




Fig. 142. Shore of Lake Ontario, near Oswego, N. Y. A wave-cut cliff which 
is being undercut and pushed back by the eroding and undermining action 
of the waves. (Oliphant. ) 



and drag the bottom, the water begins to pile up and rush 
in on the shore in an overwhelming flood that causes 
enormous loss of life and property and frequently modi- 
fies the shore-line beyond recognition. 

168. Transportation and Deposition Along the Shore. 
— The volume of water that is carried in on the shore on 
the crest of the waves is returned along the bottom in the 



202 



PHYSICAL GEOGRAPHY 



undertow which carries back with it the fine material 
formed by the grinding of the shingle on the beach. This 



7m 



J \ 



^ 



^'x 5 



^f 





^■f 


m r~w^ 


V^^^^^l 


^^^B 


\ » ' 


u /V^^^^l 


HhBBT 




'// '^ fl^^H 


W^ 


i 






: ': 



o 

■:• a 



I 



material is carried out from the beach and deposited, in 
the order of the size of the fragments, with the coarsest 



SHORE LINES 



203 



nearest the shore and the finest farthest out. Where there 
are currents at or near the shore, they affect the distribu- 
tion of the material by moving it along the beach or car- 
rying it further out in the sea. 

Besides that carried out by the undertow and the cur- 
rents, there is frequently a movement of material along 
the beach by the waves and the shore currents produced 
by the waves where they strike the shore obliquely. It is 
in this way that bars, cusps, spits, hooks, and wall beaches 
are formed. (See sections 171 to 173.) 

169. Topographic and Structural Features of Shore 
lines. — Where the waves beat against a rocky shore, they 




Fig. 144. Diagram showing a wave-cut and wave-built terrace on the shore 
line. Dotted line A, position of the former shore. B, the portion of the 
land cut away by the waves. C, the wave-cut terrace. D, the wave-built 
terrace composed of the fragments worn from the cliff by the waves and 
carried back by the undertow. 



cut away the rocks near the water-line and the shore-line 
is carried into the land by the wearing away of the rocks, 
forming the wave-cut terrace. As the terrace is cut back 
into higher land the latter is undermined and a vertical 
or overhanging shore cliff is formed, that recedes as the 
water undercuts, by the upper portion falling down, to 
be ground up and carried away. 

The material ground up by the waves at the base of 
the cliff is carried back into deeper water by the under- 



204 



PHYSICAL GEOGRAPHY 




Fig. 145. A chimney rock formed by shore erosion, Port- 
land, England. A remnant left by the wearing away 
of the surrounding rocks by the waves. 

tow and deposited beyond the edge of the wave-cut ter- 
race, making the wave-built terrace. (See figs 142 to 144.) 
Chimney Rocks. As the waves and the gravel beach 
cut into the rock cliff, some portions of it are left stand- 



SHORE LINES 205 

ing, while the waves cut down and carry away the material 
on all sides. These remnants commonly include portions 
between joint-planes and are generally for that reason 
quite angular, sometimes rectangular. From their form 
they are commonly called chimney rocks (See fig. 145.) 




Fig. 146. Remnant of a sea cave in a chimney rock on the California 
coast, near San Diego. (J. C. Branner.) 

Efect of dikes on the shore line. Sometimes an intrusive dike 
of igneous rock (see sec. 236) is more resisting to the weather 
than the surrounding rock which, crumbling away more rapidly, 
leaves the dike standing above the surface often like a great 
row of cord-wood jutting out on the beach, sometimes out into 
the water. In other places the igneous dike weathers more 
rapidly than the surrounding rock, and thus forms great chasms 
extending back into the cliff where the dike material has been 
torn out by the waves. Sometimes a chasm of this kind is cut 
across a headland, separating the end from the mainland and 
thus forming an island. There is one place on the north shore 
of Lake Superior where from a boat one may see fourteen of 
these dark-colored dikes cutting the light-colored granite rock 
within a distance of less than a mile. 

In some places on a narrow neck of land the bottom portion 
is cut through first, leaving the upper portion standing as a 



206 PHYSICAL GEOGRAPHY 

natural bridge. Several such bridges occur on the coast of Cal- 
ifornia. (See figs. 146 and 148). 

170. Sea Caves. — Sea caves are formed when the 
waves undercut the cliff more rapidly at one point. They 
commonly begin at a soft place in the rock, a fissure, or a 




Fig. 147. Deep chasm formed by a dike of igneous rock disintegrating 
more rapidly than the harder wall rock. This chasm on the shore of a 
small lake in the Adirondacks was not formed by the waves but is 
similar to many that were so formed along the shores of the Great 
Lakes and the ocean. (S. R. Stoddard.) 

joint-plane. Generally such caves are not very long, be- 
cause as soon as the opening is made, the inrushing wave 
blocks the mouth of the cave, when the air is compressed 
and acts as a cushion to protect the rocks within from the 
blow of the wave. The compression of the air and the 
sudden expansion when the wave recedes may tear loose 
some blocks and thus enlarge the cave. Sometimes the 



SHOKE LINES 



207 



land side of the cave is worn away, leaving a natural bridge 
on the shore. (See figs 148 and 149.) 




!PiG. 148. Natural bridge formed by action of the waves on the shore of the 
Pacific near Santa Cruz, Cal. The top of the bridge is part of the uplifted 
coastal plain that borders the shore. (J. C. Branner.) 




Fig. 149. Shore of the Pacific Ocean at La Jolla, Cal., showing sea caves and 
other evidences of wave erosion. (J. C. Branner.) 



OF THE 

UNIVERSITY 



208 PHYSICAL GEOGRAPHY 

Whistling caves and blow holes are formed in the sea caves, 
where an opening occurs in the roof that permits the escape of 
the imprisoned air, which rushes out often with great violence, 
producing a noise something like a steam whistle. Sometimes 
these caves are so shaped that not only the air but a column of 
water is forced out through an opening at the top, forming the 
spouting cave. (Fig. 150). 




Fig. 150. Spouting Cave in the ice on the shore of Lake Ontario at Oswego, 
N. Y. Somewhat similar caves occur in the rocks in places on the ocean 
shore. (M. S, Lovell.) 

171. The Beach.— The beach varies in character at 
different points. In the smaller coves on the headlands 
and on the bold, rocky shores, there are great accumula- 
tions of boulders and gravel. At the head of larger bays 
and along low, shores the beach is covered with sand or 
mud. The character of the material on the beach largely 
depends upon whether there is a shore cliff, the kind of rock 
in the cliff, the shape of the clift', the form of the shore 
adjoining the cliff, and the direction of the winds. 



SHORE LINES 



209 



The shingle beaches are formed at the base of rock 
cliffs where the fragments from the cliff are ground up 
by the waves. Where the incoming waves strike the shin- 
gle beach obliquely, the material is moved along the beach 
beyond the end of the cliff, forming the wall heach or 
travelling beach, which frequently extends across the mouth 
of a stream and forms a lake or causes the stream to shift 
its course beyond the end of the shingle wall. The wall 
beach is formed wherever the material is eroded from the 
cliff faster than it is carried back into deep water. 




FiQ. 151, A hooiced sand spit at Dutch Point on Lake Michigan. 
Geol. Survey. ) 



172. Spit, Hook.— Winds and shore currents trans- 
fer materials along the beach, and frequently, where there 
is a bend in the shore or a change of direction, the shore 
current may be deflected out into deep water and the 
beach accumulation be extended out from the shore as a 
point or arm, which is known as a spit. When the point 
is recurved it forms a hook, or hooked spit. The curving 

14 



210 PHYSICAL GEOGRAPHY 

of the hook is commonly due to the action of another cur- 
rent at an angle to the first one. The second current may- 
be temporary, due to a violent storm; and after it ceases, 
the spit may continue in the direction first taken until it 
meets another storm, where another hook may form, mak- 
ing in this way a series of hooks or barbs on the same spit. 
Spits are also formed in quiet waters between two cur- 
rents which carry sediment. (Figs. 151 and 152.) 




Pig. 152. A sand spit forming a bar across mouth of Floyd's Creek, Mary- 
land. (Maryland Geological Survey.) 

173. Bars.— The spit may form at the headland at 
the opening of a bay, or it may form along the sides of a 
bay. In time, if it is not checked by a strong river or 
tidal current through the bay, it may extend entirely 
across, joining the land on the opposite side, when it is 
called a har, sometimes a hay bar to distinguish it from 
bars formed in other ways. 



SHORE LINES 



211 



Small islands along a shore are frequently tied or joined to 
the shore by a bar which may have started as a spit from the 




Fig. 153. Tie-bars connecting the mainland of Italy with Monte Argen- 
tario Island. (See Boston Bay, Mass., Sheet U. S. Top. Atlas for 
similar example on New England Coast.) 



island or from the shore or from both places. There may be 
either one or two, sometimes more of these bars tying the island 
to the shore. They are formed where the islands lie near shore 



212 



PHYSICAL GEOGRAPHY 



and there is sufficient rock waste from them. They are some- 
times called tie-bars because they tie the island to the shore. 

174. Barriers.— Where there is a stretch of shallow 
water off shore, there is sometimes a violent agitation of 
the bottom sand and mud by waves which form the break- 
ers at some distance out from the shore. The meeting, 
in these muddy waters, of the waves from the sea on one 




foreland. 



of Lake Champlain, Y/ 
t»uiall sand barrier, lagoon 
(H. M. Brock.) 



iiiia loiiy: sand spil or uuspate 



side and the undertow from the land on the other side, 
checks the velocity of each, causes a deposit, and builds 
up an off-shore ridge or bar, which in time reaches the 
surface, above which it is built by the waves and wind. 
Such an off-shore bar is called a harrier. The shallow 
water or lagoon behind the barrier is in time filled by the 
sediment carried in by the rivers, the sand and dust car- 
ried in by the winds, both aided materially by the accu- 



SHORE LINES 



213 



mulations of vegetable and animal matter. After the 
filling of the lagoon, the former barrier becomes the beach 
of the new shore line and another barrier develops, in this 
way extending the land area into the sea. A barrier beach 
is formed where the water is too shallow for the waves to 
attack the shore. (For description of the coral barrier 
reef see sec. 181). 

175. Shore Terraces.— Terraces are formed along the 




Fig. 155. Sand terraces on the north shore of Lake Superior. Drawn 
to scale from barometric measurements by the author. The terraces 
are known to the Indians as "Manabozho's stair steps." 



shore, by the elevations of the land which carry the first 
beach above high water and expose a new surface on 
which another beach is formed. The elevated beaches 
form in some places boulder terraces, in others sand or 
gravel terraces, in others bed-rock or wave-cut terraces, 
(See figs 155 and 155a.) 

176. Irregular Shore Lines.— An irregular shore line 
is generally caused by a migration of the shore line land- 
ward, due either to subsidence of the land area or to the 
erosive action of the waves. The beating of the waves on 
a new shore line will first wear away the softer rocks, or 
the more exposed ones, forming indentations or bays, with 



214 



PHYSICAL GEOGRAPHY 



the harder rocks standing out as headlands and promon- 
tories. Irregularities in the rocks on the headlands cause 
caves, natural bridges, chimney rocks and islands. (See 
figs. 156, 157, 146, 148 and 149.) 




am. 155a. Shore terraces on the moufctain bordering Great Salt Lake, 
Utah. The terraces mark former shore Ijnes of the extinct Lake Bonne- 
ville. Notice the desert vegetation in the foreground. Salt Lake 
visible in the background on the right. (D. T. McDougal.) 

The most irregular shore lines are produced by the sinking 
of the land and the advance of the sea, in which case the sea 
extends up the valleys, sometimes long distances, forming bays, 
estuaries or fiords, that is, drowned valleys. The hills bordering 
the valleys form headlands on the submerged area. What were 
monadnocks and peaks before submergence, become islands along 



SHORE LINES 



215 



the new shore. In the drowning of the lower portion of the 
valleys the rivers are dismembered and the tributaries flow di- 
rectly into the arm of the sea. (Study the Boothbay sheet of 
the U. S. Top. Atlas.) 




Fig. 156. Irregular shore on the Oregon coast, formed by wave erosion. The 
more resistant rocks form the headlands and islands, which are the 
remnants of the land area cut away on each side by the waves. (J. E. 
Kirkwood.) 



177. Regular Shore Lines.— Regular shore lines are 
formed in several ways: (1) By the uplift of the land, 
which causes the shore line to miove seaward from the old 
land border on to the newly uplifted coastal plain, pro- 
ducing a quite regular coast-line. This regularity of shore 
line is due to the smoothness of the sea bottom. 

(2) By the building of sand reefs parallel to the 
shore, causing a movement of the shore outward to the 



216 



PHYSICAL GEOGRAPHY 




SHORE LINES 217 

reef. This filling in of the lagoon and marshes inside the 
bar, removes the inequalities. 

(3) By the waves cutting off the headlands and filling 
up the bays, thus making a more regular coast line. 

(4) By delta deposits at the mouth of a river flowing 
into a bay finally filling the bay and thus straightening 
that portion of the coast. The extension of the bay-bars 
across the bay and the tie-bars between the islands and 
the mainland tend to straighten the coast line. 

SHORE LINES MODIFIED BY LIVING FORMS 

The shore line is sometimes greatly modified by tKe 
accumulation of organic matter, both vegetable and ani- 
mal. 

178. Vegetable. — In tropical regions one of the most 
important plants that affect the shore line is the Mangrove 
tree, one of the very few trees that flourishes in salt water. 
The seed often sprouts while still attached to the branch, 
and sends forth a long radicle or root stem which fre- 
quently extends to the mud at the bottom, takes root and 
from the top a new stem or trunk is formed. From the 
trunk many spreading roots extend to the bottom, and many 
branches form at the top, some extending downward to 
start new roots and new trunks, until a single parent tree 
is surrounded by a small grove. Some of the fruit drops 
off and floats away on the water with the long radicle ex- 
tending towards the bottom until it finally becomes at- 
tached to the bottom mud and starts a new tree and a new 
grove. 

The network of roots catches and holds drift material 
and mud, until a solid embankment is built up. The shal- 
low lagoon between it and the mainland in time fills with 
accumulated vegetable and animal matter and mud de- 



218 PHYSICAL GEOGRAPHY 

posits. A continuation of this process extends the shore 
line seaward wherever the building up is faster than the 
destruction by the waves. The shore plain built in this 
way is apt to be wet and marshy. The mangrove is very 
abundant on the coast of Florida. (See fig. 158.) 




Fig. 158. Mangrove trees on the shore of a lagoon at low tide. Gilbert Group 
of coral islands in the Pacific Ocean. The area is covered by the sea at 
high tide. (U. S. Fish Com.) 

Eel and marsh grass. The mangrove does not grow north of 
Florida, but the low-lying plains bordering the shallow water 
along the northern shore of the United States are extended sea- 
ward in a somewhat similar manner by another kind of vegeta- 
tion. The eel grass grows over the shallow bottom below low 
tide, where it acts as a trap to catch the mud stirred up by the 
waves, until the bottom is raised to low tide surface-level, when 
the marsh grass takes possession and aids in the upbuilding 
process up to or sometimes above high tide. The repetition of 
this process causes the extension of the marsh grass plains 
seaward. 

179. CoraL—One of the most prominent of all the 
land builders along the shores is the coral polyp, an ani- 



SHORE LINES 



219 



mal which secretes carbonate of lime that it extracts from 
sea water. It grows in such multitudes that even though 
a single coral polyp secretes but a small quantity of lime 





t'' 




^^^ 


ly ^Xr 




^^ *«^^ 




^ 




^^^^? 


i 








mm 






m 


^»^ 



Fig. 159. A coral colony showing the polyps opened. The polyps 
somewhat resemble flowers. (Smith. Inst.) 



the aggregate is something astounding. The Great Bar- 
rier Reef off the coast of Australia is more than 1,000 
miles long and contains a mass of limestone probably 



220 PHYSICAL GEOGRAPHY 

equal to any of the great limestone beds extending through 
the central and eastern United States. 

The reef-building coral flourishes only in tropical seas where 
the winter temperature of the waters does not fall below 68 de- 
grees F. It does not grow above the surface of the salt water 
at low tide nor at depths greater than one hundred feet. It does 




Fig. 160. Coral Head or Mushroom. Beach north of Hepuhepuama, 
Makemo atoll, in the Pacific Ocean. (U. S. Fish Com.) 

not grow in muddy waters, hence is not found at the mouths of 
rivers. It grows best in waters that are violently agitated by 
the waves and currents, hence, it is not found in great abun- 
dance inside the atolls, but flourishes on the outside in the midst 
of the surf and breakers. The reason for this is that it needs a 
constant supply of food, oxygen, and carbonate of lime, which 
is soon exhausted in the lagoon, but is constantly renewed by the 
moving waters in the breakers. 



SHORE LINES 



221 



Thick coral beds. While the coral does not grow at 
depths greater than 100 feet, some of the reefs appear to 
be several thousand feet deep ; at least the dredge brings 
up dead coral from that depth off the shore of the reefs. 
This is accounted for in two ways : ( 1 ) The coral grow- 
ing outward from the reef at the top forms overhanging 
masses which break off from their weight or are broken 
off by the weaves and slide down the steeply inclined 
sea-bottom into the deep water; or (2) the bottom sub- 
sides as the coral is growing and the coral that grows 




Fig. 161. Illustrating the development of an atoll from a fring- 
ing coral reef. ss, former sea level. F." fringing reef, 
s' s', subsequent level of the sea after sinking of the island. 
BB, barrier reef, s" s", sea level after further subsidence 
when AA is a coral atoll surrounding a lagoon. (Darwin.) 



near the surface is carried down into deep waters by the 
sinking of the bottom. This may continue indefinitely 
without killing the coral at the top, providing the sinking 
does not take place more rapidly than the coral grows. It 
may be slower, but not faster. (See fig. 161.) 

180. Coral reefs. — The coral deposits attached to the 
shore form fringing reefs, such as those on the Bahama 
Islands. Those that occur out from the shore at dis- 
tances varying from a fraction of a mile to several miles, 
form harrier reefs, such as the keys off the coast of Plor- 



222 PHYSICAL GEOGRAPHY 

ida. Such reefs are separated from the mainland by a 
lagoon which is frequently deep enough for a ship channel. 
By subsidence of the island a fringing reef may be- 
come a barrier reef and in time an atoll, or circular reef 
inclosing a body of salt water or lagoon. Atolls may al- 
so be formed by coral growth on the rim of an extinct 
volcano or on any sea bottom less than 100 feet deep. 
Whitsunday, Caroline and many other islands in the Pa- 
cific are atolls. (See fig. 162.) 




Pig. 162. Portion of Pinaki Atoll and barrier reef. Paumotu gn^oup, 
Pacific Ocean. The semi- circular light band of breakers marks the 
position of the barrier reef surrounding the atoll. (U. S. Fish Com.) 

181. Fossil Reefs.— There are fossil coral reefs in the 
limestone beds at Syracuse, New York, at the falls of the 
Ohio River at Louisville, Kentucky, at Chicago, Illinois, 
and many other places in the United States, signifying 
that these areas were at one time covered by the sea, with 
conditions favorable to coral growth. What does this in- 
dicate regarding the climate of central and northern United 
States in times past ? 



SHORE LINES 



223 



182. Limestone from Other Animals than Coral.— 

Growing in the same waters with the corals, is a great 
variety of other animals and plants, many of which secrete 
carbonate of lime, while others deposit silica. Such are 




Fig. 1().3. Shell beach, Lagoon of Pinaki, Paumotu group, Pacific 
Ocean. The beach is composed of shells of mollusks which in 
time will form a bed of limestone similar to many that now 
occur on the continents. (U. S. Fish Com.) 



the different kinds of molluscs, crinoids and sponges. 
There are also many microscopic forms. The aggregate 
remains of the multitude of different forms are mingled 
with the corals in the formation of the coral limestone 
beds. In many places extensive beds of limestone or sili- 
cious rock are formed by the molluscs and other forms of 
life without any reef -building coral. (Fig. 163.) The 



224 PHYSICAL GEOGRAPHY 

coquina limestone now forming in this way along the Flor- 
ida coast is used to some extent for building stone. 

183. Lake Shores.— Lake shore lines are similar to 
ocean shore lines in many ways. The lake waves are 
neither so large nor so strong as the ocean waves, hence 
the erosion is not so rapid. The water, except that in 
the salt lakes, is not so dense, hence sediment is not car- 
ried as freely as in the salt waters. In the northern lati- 
tudes the lakes, except a few of the largest ones, freeze 
over in the winter season, and are not subject to active 
erosion on the shore by the winter winds. The ice, while 
protecting the shore from the winds, becomes an active 
agent of erosion in itself. The expansion of the ice due to 
changes in temperature causes it to push against the shore 
with great force. When the frozen surface is broken up by 
warm weather the blocks are driven or. the shore by storm 
winds. 

There is no coral in fresh water, and most of the other forms 
of life common in the ocean do not occur in the lakes, which 
have a fauna and flora of their own. In the larger lakes the 
living forms affect the shore life very little; but in many small 
lakes, vegetation accumulates along the shore and forms marshy 
plains which in time cover the whole lake basin. Some of the 
small lakes are bordered by plains composed of marl, which con- 
sists of the remains of animals and plants that grew in the lake 
in sufficient quantities to partly, sometimes entirely, fill it. 

184. Fossil Shore Lines.— How may we recognize a 
former shore line after the body of water which caused 
it has disappeared? Many of the features explained on 
the preceding pages are characteristic of shores and not 
found elsewhere; hence a recognition of these features 
means the recognition of a former shore line. 

North of Lake Superior, at different elevations on the hills, 
are boulder beaches similar to those at the water's edge to-day. 
At other points are the wave-built sand-terraces, similar to those 



SHORE LINES 



225 



forming on the present shore. These old beaches are at differ- 
ent elevations above the lake, some less than one hundred, some 
more than three hundred feet above the water. (See fig. 155.) 




Fig. 164. Chazy limestone, Chazy, N. Y. Showing the fossil shells, that 
are the remains of animals buried in the mud in the margin of the 
shallow sea that covered that area millions of years ago. (H. M. Brock.) 

Along the coast of California in several places are wave-cut 
terraces many feet above the sea, indicating a recent elevation 
of the land that carried the former shore line high above the sea- 
level. (See fig. 148.) 

Besides the cobble beaches and sand terraces, other shore 
features that may often be recognized on fossil lakes are wave- 
cut cliffs, terraces, bars, spits, hooks and deltas. 

Examples. Surrounding Great Salt Lake, in places 
some miles from the lake, is a prominent shore line, or 
rather a series of them indicating the levels of a former 
great lake which has been called Lake Bonneville. (See 
lig. 155a.) 
. Lake Agassiz in central North America, Lake Passaic 
15 



226 PHYSICAL GEOGRAPHY 

in New Jersey, and Lake Iroquois in New York are other 
fossil lakes which have been recognized by their shore 
lines. (See fig. 86.) 




Fig. 165, Fossil boulder beach on north side of Adirondack Mountains, Covey 
Hill, Canada, 480 feet above tide. Similar boulder beaches bordered by 
shore cliffs occur at lower levels. (H. M. Brock.) 

185. Sand Dunes.— On sandy shores where the pre- 
vailing winds are from the water, the sand forms great 
ridges or dunes, varying in height from a few feet to sev- 
eral hundred feet. In an open country the sand ridges 
are generally formed at right angles to the direction of 
the winds. Where there are local obstructions, the small 
dunes are parallel with the direction of the winds. The 
blowing inland of the sand from the shore often aids ma- 
terially the cutting back of the shore line by the waves. 

On the coast of southwestern France the prevailing west 
winds have blown the sand inland, forming dunes several miles 
in width, which have covered farms and villages as they were 



SHORE LINES 



227 



driven forward by the winds. The further progress of these 
dunes has been checked by planting trees over and in front of 
them, which in some places not only stop the drifting dunes but 
even form productive forests. (Fig. 167.) 




Fig. 166. iSaiid duiit- at Biggs, Oregon, covered with wind ripples. Wliich 
way was the wind blowing that formed the dune ? Notice the alluvial 
terraces in the background. (U. S. Geol. Survey.) 



The eastern and southern shores of Lake Michigan contain 
many sand dunes of great size, which have in places made in- 
roads on the fertile farm land. Attempts have been made to 
check their further progress by the growth of grass and trees. 
Similar but smaller dunes occur in large numbers in New York 
State at the east end of Lake Ontario. They are very abundant 
in many places along the Atlantic coast, especially in the Caro- 
linas' 

Sand dunes vary greatly in height. On the coast of Holland 
some are two hundred and sixty feet; on Cat island, one of the 
Bahamas, nearly four hundred feet; and on the western coast of 
Africa near Cape Bajador, more than five hundred feet high. 

Sand dunes are also common features on many sandy areas 



228 



PHYSICAL GEOGRAPHY 



remote from and independent of shore lines. They are very- 
abundant in desert and semi-arid areas and along many river 
flood plains in humid areas. (See fig. 222.) 




Fig. 167. Diagram showing method of checking drifting sand. A solid 
bj^rier, A, causes a dune in front of it. An open but rigid barrier, B, 
causes the dune to form in and finally over it. An open flexible barrier. 
C, causes the dune to form behind. The sand moves from left to- right 
in the diagram. (After the U. S. Dept. of Agriculture.) 

186. Harbors.— The most important economic fea- 
tures along the sea coast are the harbors, or places of 
shelter for vessels in time of storm. In some places there 



SHORE LINES 



229 




Fig. 168. Dune Park, Ind. Advancing front of a moving dune, bury- 
ing forest and swamp vegetation. (H. C. Cowles. ) 









WM 


'.-■^^''^■j^H 






T 


^Ij^^^k 






^n'' ■i'-^MmJ 


j^i^^^mj^^^B 






B|g 








1 



Fig. 169. Dune Park, Ind. Roots of the cottopwood tree exposed by the 
action of the wind. In time the tree will be completely undermined 
and overthrown. (H. C. Cowles.) 



230 PHYSICAL GEOGRAPHY 

are long stretches of coast line without any cities, because 
there is no harbor for the vessels. 

The conditions necessary for a good harbor are: (1) 
protection from incoming heavy waves, (2) an open, deep 
channel extending from the anchorage to the open sea, (3) 
water deep enough to permit the vessels to approach close 




Fig. 170. Fiord Harbor on coast of Norway. Many snch harbors lie well 
inland, sheltered from the winds by high hills. Naero Fiord, Norway. 

to the shore line to facilitate loading and unloading, (4) 
location convenient to natural highways into the interior, 
(5) roomy enough to accommodate many vessels without 
interference, (6) good bottom for anchorage, and (7) 
absence of strong river or tidal currents. 

(1) Delta harbors, on the delta of a great river, have the ad- 
vantage of access by water to the great fertile plains of the in- 
terior of the continent, but they are often hampered by the 
diflaculty of keeping a ship channel open and free from the 
mass of the mud carried in by the river. 



SHORE LINES 231 

(2) Estuary delta harbors are on drowned rivers where the 
sea has entered the lower part of the valley and a new delta has 
formed at the head of the bay or estuary. 

(3) Fiord harbors differ from the preceding in being deeper, 
and generally lying in rock depressions with less soil on the 
bordering hills than commonly occurs along the bays or estua- 
ries. The fiords represent the deep ice-worn channels of glacial 
origin, and hence are found only in high latitudes where the 
glacial streams run into the sea. Their origin accounts for the 
bare rock walls and scarcity of mantle rock. They are abundant 
on the coast of Norway. (Fig. 170.) 

(4) Mountain ranges that project into the sea frequently 
have troughs or depressions below sea level which may be util- 
ized as harbors. Such is the western end of the Pyrenees in 
Spain, and the peninsula and islands of Greece. Sometimes the 
mountains are parallel to the coast, and the harbor or harbors 
may lie inside the first range, as San Francisco Bay and many 
similar land-locked areas along the coast of Washington and 
Alaska. 

(5) Glacial moraine deposits along a sea coast sometimes 
form protected harbors. 

(6) Lagoon and sand-bar harbors occur on almost all sandy 
shores where there is a long stretch of shallow sea bordering 
the coast. The waves build up a barrier at some distance off 
shore, and the lagoon between the bar and the shore, where deep 
enough, furnishes safe anchorage for vessels. Most of the la- 
goons of this kind are not deep enough for the modern ocean 
steamship, unless deepened artificially by dredging, but they 
serve a useful purpose for the smaller coasting vessels. 

(7) Sand-spit harbors are similar in some respects to the last 
mentioned, but in the spit the sand is drifted along the shore 
until in drifting past the headland at the entrance of a bay the 
spit is carried part way or perhaps entirely across the bay, thus 
making a safe anchorage in the bay behind it. Provincetown, on 
Cape Cod is an example of this class. 

(8) Volcanic crater harbors are formed by a breach in the 
rim of a volcanic crater that occurs near sea level on an island 
or the border of a continent. The accompanying view (fig. 171) 
shows such a crater at the village of Ischia, on the island of 
Ischia, near Naples. The notch in the rim of the crater permits 



232 



PHYSICAL GEOGRAPHY 



small vessels to enter and find a snug harbor in the crater of 
the extinct volcano. 

(7) Coral reefs and atolls furnish many much-needed harbors 
in the tropics. The lagoon inside of the barrier reef or on the 
interior of an atoll frequently furnishes a good harbor for large 
as well as small vessels. Often the entrance to the coral har- 
bor is narrow, intricate and dangerous. Biscayne bay, on the 
east coast of Florida, is an example of a coral barrier-reef har- 
bor; and Hamilton, on the Bermudas, is an example of an atoll 
harbor. Both of these types are much more numerous in the 
Pacific than in the Atlantic ocean. 




Fig. 171. A volcanic crater harbor on the Island of Ischia in the Bay 
of Naples, Italy. A notch in the rim of the crater forms an opening 
through which boats pass to the open sea. 

187. Economic Importance of Haxbors.— The pres- 
ence or absence of good harbors has much to do with the 
location of cities and the commercial prosperity of the 
adjacent region. The location of San Francisco is not an 
accident. It has one of the best harbors on the Pacific 



SHORE LINES 233 

coast, which is at the same time a connecting link between 
the ocean and the great fertile interior valley of Califor- 
nia. New York City on the east coast is the metropolis of 
the United States mainly because of its good harbor, lo- 
cated at the natural doorway into the interior of the con- 
tinent through the Hudson Valley and the Great Lakes. 
Boston, Philadelphia, Baltimore, Washington, and all the 
prominent cities on the eastern coast owe their locations 
mainly to their good harbors. 

In a few places there are sufficient attractions to cause 
the growth of a town or city where there is no harbor, and 
the commerce is carried on under great difficulties. Nome 
City on Cape Nome, Alaska, is an example. Great quan- 
tities of gold are found in the sands and gravels of the 
sea shore and the inflowing streams. People come to get 
the gold and must have food, clothing, houses, machinery, 
etc., which are brought, by ships on the ocean, but they 
cannot get close to the shore because of the shallow water. 
The goods and passengers are unloaded by lightering, that 
is, a lighter vessel or boat takes a portion of the freight 
through the breakers and shallow water to the shore. 
Sometimes horses and cattle are thrown into the water and 
made to swim ashore. In time of a storm the lighter can- 
not get through the breakers, and the large vessels must 
wait until the storm subsides, which may be several days. 
In a violent storm it must put out to sea to avoid being 
driven aground in the shallow water and destroyed by 
the waves. 

In the absence of a natural harbor, sometimes an arti- 
ficial harbor is constructed at a great expense. This is 
done by building a wall or breakwater out from the shore, 
inclosing water deep enough to float the vessels. The 
breakwater is so-called because on it the heavy storm- 
waves from the open sea break and lose their power to 



234 PHYSICAL GEOGRAPHY 

injure a vessel lying safely in the calm waters behind the 
wall. The harbor for the city of Los Angeles at San 
Pedro on the Pacific Coast is a type of this class. See the 
Oswego sheet of the U. S. Topographic Atlas for example 
of artificial harbor at Oswego. 



REFERENCES 

Shaler, Beaches and Tidal Marshes, Natl. Geog. Mon., Amer. 

Book Co. Natural History of Harbors, 13th An. 

Rept. U. S. Geol. Survey. 
Gulliver, Shore Line Topography, Proc. Am. Acad. Arts and 

Sci., Vol. 34, Jan., 1899. 
Gilbert, Features of Lake Shores, 5th An. Rept. U. S. Geol. 

Surv. p. 75. 
Darwin, Structure and Distribution of Coral Reefs, Appleton 

& Co., New York, 1889. 
Dana, Corals and Coral Islands, Dodd, Mead & Co., New 

York, 1895. 



CHAPTER VII 

THE LAND 

The land or solid portion of the earth, has many fea- 
tures in strong contrast with the water, or liquid portion, 
and the atmosphere, or gaseous portion. It is subject to 
change like the other parts. The mountains, the plains, 
and even the rocks themselves undergo cycles of change, 
each with its own life-history, extending over a very long 
period of time. 

So slow are the changes that to the casual observer the 
mountains were always mountains and will ever remain 
such, but the geographer sees the mountains and hills in 
process of growth and decay, and to him they are living 
objects of interest as he studies their varied changes. So 
with the plains, plateaus, valleys, volcanoes and other 
natural features. He studies carefully the variations 
going on and learns that by properly interpreting these 
he can trace out the birth, growth, maturity, decay and 
disappearance of even the ''everlasting hills." Their 
previous history is recorded and preserved in the rock 
strata which have been wisely compared to the stone 
leaves in the book of nature from which the geologist reads 
the history of the earth and its development. The study 
of this history is properly the province of Geology. It is 
the province of Geography to interpret the elements of 
this history and study the ways in which it is made and 
recorded. 

188. Divisions.— The two main continents have been 
named the Eastern, comprising Eurasia and Africa, and 
the Western, comprising North and SoutH America. 

235 



236 PHYSICAL GEOGRAPHY 

Australia is a tliird continent much smaller than either of 
the other two; and probably Antarctica, of which little is 
known, may prove to be a fourth. Continents are dis- 
tinguished from islands; first, by size, being much larger; 
and second, by structure, since, except the coral atolls, 
the interior of islands is high land sloping to the sea, while 
the interior of the continents is lowland consisting of 
great river basins. The high land and great mountain 
ranges occur mostly on or near the margin of the continent, 
and not in the interior, as in the case of islands. 

189. Islands. — Islands may be divided into two 
classes, continental and oceanic; the first includes those 
that lie near the continents on the continental shelf and 
hence are surrounded by comparatively shallow water; 
the second, those that rise out of the deep waters of the 
ocean basins. 

Continental islands are of two kinds; (1) those built 
up on the shallow ocean bottom by corals or by currents, 
waves, and wind, such as the sand barriers along the 
Atlantic coast; (2) Remnants of the continent left above 
the sea level by an advancing shore line. The hills of the 
old shore become islands along the new shore. Such are 
the islands along the coast of Maine. Those formed in 
the first way are low and sandy with sandy shores. Those 
in the second are generally rocky and bordered by rock 
cliffs. 

190. Distribution of the Land.— Tt may be observed on a 
globe or a map of the continents that much more than half of 
the land area lies north of the equator. If we should divide the 
globe into two hemispheres by taking London, the greatest cfty 
in the world, as a center of one of them, it would be seen that 
nearly all of the land (about nine-tenths) is included in the Lon- 
don Hemisphere, while the other is largely a water-area. Is 
there any significance in the location of London in the center 
of the land hemisphere? 



THE LAND 



237 



Since it is the land masses that divide the ocean into its 
different parts, the unequal distribution of the land causes an 
unequal division of the oceans; thus the Pacific ocean which lies 
mainly in the water-hemisphere is much larger than any of the 
other oc"eanic divisions. 

191. Importance of the Land.— To the existence of 
dry land is due the possibility of all forms of land life. 




Water Hemlspbdre. hand Hemlspliere. 

Fig. 172. Division of the earth into two hemispheres, one of which contains 
nearly all the land. London is near the center of the land hemisphere. 
Where is tHe center of the water hemisphere ? 

It is possible that a large part of the life in the sea is 
dependent on the occurrence of land, since much of the 
material to supply the marine life comes from the land. 
To the land areas are due likewise the direction, intensity, 
and in large measure the existence of the ocean currents 
so important to life both in the sea and on the land. 
They also influence in large measure the movements of the 
atmosphere and the distribution of moisture in the form of 
rain and snow. Imagine what a monotonous and dreary 
planet this earth would be if the entire surface were cov- 
ered with water as it would be if the rock surface were free 
from elevations and depressions. 



238 



PHYSICAL GEOGRAPHY 



192. Topography of the Land Compared With the 
Sea Bottom. — In its broad, general features, the land 
areas somewhat resemble the ocean floor in reverse order. 
The continents correspond in a way, above the ocean level, 
to the ocean basins below except that they are smaller. 
The great plateaus on land have their opposites in the anti- 
plateaus or deeps of the ocean. Here the analogy ceases, 
as there is no erosion over the ocean bottom to correspond 
to the multitude of valleys and steep hillsides on the land. 
The widespread monotony of the ooze-covered plains of 
the ocean bottom is replaced on the land by an ever vary- 
ing diversity of landscape produced by the carving action 
of the rainfall and the streams on the uplifted plateau and 
mountain masses. 



\^a..-s-^^"^'^ 




Potassium 1 . Z" 
All others not \ ./<>■ 



Fig. 173. Diagram showing the approximate percentage of the common ele- 
ments that form the known part of the earth. More than half of the 
oxygen is combined with the silicon. (Hessler and Smith.) 

193. The Composition of the Earth's Crust.— The 

solid portion of the earth contains a greater variety of 
chemical elements than the atmosphere or the hydro- 
sphere. About eighty different elements have been recog- 
nized by the chemists, but only a few of these form an 



THE LAND 239 

appreciable part of the rocks and the minerals of the 
earth's crust. One element, oxygen, forms about half of 
all the known part of the earth, including the air, water 
and land. The other most common elements are silicon, 
aluminium, calcium, magnesium, iron, sodium, potassium, 
carbon, hydrogen, and nitrogen. The elements enter into 
many different mineral and rock combinations, but again 
the bulk of all the rocks is made up of a comparatively 
small part of the hundreds of known minerals.* (See fig. 
173.) 

194. Minerals. — A mineral is a portion of inorganic 
homogeneous material produced by natural means, having 
a definite or nearly definite chemical composition and gen- 
erally having a crystalline structure. Most minerals are 
crystalline and many have a definite crystal form, but 
many occur in rock masses where the crystal form does not 
appear, while some few have not even crystalline texture. 

Minerals are distinguished from each other by a care- 
ful comparison of all the physical properties such as hard- 
ness, color, color of the powder or streak, crystal form and 
habit, cleavage, luster, optical, electrical, and magnetic 
properties, all of which can be learned satisfactorily only 
by the study of the mineral specimens. They may also be 
distinguished by their chemical properties which may be 
tested by the use of the blowpipe with different reagents. 
Minerals are classified commercially according to their 
uses, and scientifically according to their composition. 

Suggestions: In studying the hardness of minerals it is cus- 
tomary to select a certain number, generally ten, and arrange 
them in the order of relative hardness, as a scale for com- 

*It is not necessary to study all the minerals described in the following 
pages. The student should study the minerals in his laboratory collection 
and use these pages for reference. Do not study the text on minerals with- 
out the minerals, but in connection with them. 



240 PHYSICAL GEOGRAPHY 

parison. The ones commonly selected are (1) talc, (2) gypsum, 
(3) calcite, (4) fiuorite, (5) apatite, (6) orthoclase, (7) quartz, 
(8) topaz, (9) corundum, (10) diamond. By comparing any other 
mineral with these its hardness can be designated by a number 
corresponding to that given ti the mineral with which it agrees 
in the scale. For example, a mineral that would scratch calcite, 
but not fluorite would be marked 4 in the scale commonly writ- 
ten H (=) 4. The powder or streak may be obtained by rub- 
bing the mineral on unglazed porcelain or by scratching it with 
a file. 

All crystal forms of materials may be divided *into six groups 
or systems, but the determination of these involves more knowl- 
edge of crystallography than can be given here. Compare the 
crystals as to the number of faces, the number of the same kind, 
the shape of the faces and the arrangement of them. The dif- 
ferent kinds and degrees of cleavage and luster can be learned 
by comparison of the different minerals. 

Rock-forming minerals. The most important rock- 
forming minerals are quartz, the feldspars, micas, horn- 
blende, augite, calcite, dolomite, serpentine and kaolin. 

195. Quartz, the oxide of silicon (SiO^ ), forms one of 
the essential minerals in granite, the bulk of the grains in 
most of the sandstones, and a part of some other rocks. It 
is one of the hardest of the common minerals, 7 in the 
scale, readily scratching glass. It crystallizes in six-sided 
prisms with pyramids (See fig. 174). It has a conchoidal 
fracture, and commonly a vitreous luster, some varieties 
have ' a waxy luster and others a dull luster. While it 
occurs in nearly all the different colors, in the granites 
and sandstones it Is usually gray, white, or colorless. The 
streak, difficult to obtain, is white or gray. In the mineral 
form it is used commercially in the manufacture of glass 
and porcelain. Some varieties, as rock crystal, amethyst, 
jasper, and chrysoprase are used as gems. Some sand- 
paper is made from ground quartz. Compare quartz with 
calcite, feldspar, fluorite, and selenite. 





/ Cute. 



^' 



/d fyritohejtoti. 
Rrite 




5 OctotieJron. 
Mametite^ ffhte, 
Fluotite. 






Heyaaona/ 
4-. fPismanc/lframicl. S^Qa^rfz Crystal. 
Quartz. 



n\ 




6. Ocwenohec/tcn. 
CJcite-DojfootIf Sia^r. 




tiJ 




7 Rhombohec/ron. 
Cleavage form ofCalcitc. 



0- nflonoclinic. y. Irafsezohetfton. 

Orthoclase feUspar. Gdrhet. 

Fig. 174. Some of the common crystal forms. The same mineral may occur 
in different forms, as pyrite in the first three above, but for the same 
mineral these are always in the same one of the six systems. The three 
forms given for pyrite are in the isometric systems. The two forms for 
quartz, Nos. 4 and 5 are in the hexagonal svstem. 
16 



242 PHYSICAL GEOGRAPHY 

196. Feldspars are divided into two classes, ortho- 
clase_ and plagioclase. The first occurs in granites and 
syenites, the other in the diorites and gabbros. The ortho- 
clase contains potash combined with silica and alumina, 




Fig. 175. Half of a large feldspar (al])ite) crystal from Cabot, Vt., 4 feet 
long 2y2 feet wide. The feldspai-s and quartz sometimes form very 
large crystals. (C. H, Richardson, Vt. Geol. Survey.) 

while plagioclase contains soda or lime or both in place of 
the potash. The feldspars are not quite so hard as quartz 
being one less in the scale, but are still hard enough to 
scratch glass. They differ from quartz in having bright 
cleavage surfaces in two directions at right angles or 
nearly so. Feldspar becomes dull on weathering as it 
disintegrates and finally crumbles into a soft, clayey mass. 
Feldspar is commonly white, gray, or pink in color. It 
is quarried in New York, Pennsylvania, New Hampshire, 
and elsewhere and is used in the manufacture of porcelain 
and chinaware. (See fig. 175). 



THE LAND 243 

197. Micas are characterized by the extremely thin 
plates or scales into which they may be separated, due to 
the perfect cleavage. There are several different varieties, 
of which muscovite and biotite are the most common. 




VlQ. 17C. Feldspar quarry near Elam, Pa., August, 1898. Feldspar 
occurs in commercial form in veins or dikes in igneous or metamorphic 
rocks. It forms a large part of the granite rocks. 

Muscovite, the so-called isinglass, is colorless in thin pieces 
when pure. It is used for electrical purposes, lanterns, 
stove and furnace doors, as a lubricant and for decorative 
purposes. It occurs in granite, syenite, and in some 
schists, and sandstones. Biotite is black or dark green in 
color and occurs in granite, syenite, and some schists, also 
in diorite and some of the other dark-colored igneous rocks. 
Muscovite has a composition somewbat similar to that of 



244 PHYSICAL GEOGRAPHY 

orthoclase ; in biotite, iron and magnesia replace the potash 
of the muscovite. 

198. There are several varieties x)t amphihole, the most com- 
mon of which is hornhlende, a black mineral occurring in 
syenite, diorite, some granites and schists. It may be dis- 
tinguished from biotite by not separating in thin scales. One 
form of the fibrous mineral, asbestos, is a variety of hornblende. 
Another form of asbestos is a variety of serpentine. It is the 
latter that is used extensively for making fire-proof cloth, and 
covering steam pipes and boilers. 

199. Augite, (the most common of the pyroxenes) is a dark 
green mineral which occurs in diabase, basalt, and gabbro. 
Augite and hornblende are important rock-making minerals com- 
posed of silicates of iron, magnesia, and lime. Varieties of each 
differ in color but where they form a large part of the rock mass, 
the augite is dark green and the hornblende is black. The 
augite is commonly in short, thick crystals or irregular masses, 
hornblende usually in long, slender crystals, sometimes finely 
fibrous. 

200. Calcite is composed of the carbonate of calcium 
(CaCO^) and forms the bulk of the limestones, marbles, 
and chalk deposits. It forms a large part of marl, of 
shells of all kinds, most of the coral, extensive deposits in 
caves and about lime springs and occurs in veins or fissures 
in different kinds of rocks. The limestones and marbles 
besides having extensive use as building and ornamental 
stone are used for making quicklime and cement and hence 
they form the base of most of the mortars in building 
operations. Calcite is one of the most useful of all the 
minerals and fortunately is very widely distributed. 
Compare calcite with dolomite, quartz, and fluorite and 
point out the differences, telling how you would distin- 
guish them. When pure it is colorless to white; impure 
varieties occur in all colors— red, black, blue, gray, and 
yellow are abundant. It cleaves readily in three direc- 
tions into rhombohedrons. Clear forms, Iceland spar, 



THE LAND 



245 



show double refraction. It effervesces freely in dilute 
acid. (See figs. 177 and 174.) 

201. Dolomite is the double carbonate of lime and 
magnesia, and hence differs from calcite in having part 




Fig. 177. Travertine quarry at Bagn^, near Rome, Itaiy. The 
rock is calcite deposited from solution in the spring water. 
From this quarry the rock was obtained for the Coliseum, 
St. Peters, and other buildings in Rome. (J. C. Branner.) 

of the lime replaced by magnesia. It frequently resembles 
calcite so closely, especially in many limestones and 
marbles, that it is difficult to distinguish from it. It may 
commonly be distinguished by adding quite dilute cold 
hydrochloric acid, in which the calcite will effervesce 
vigorously and the dolomite but little, if at all, until the 
acid is heated. 

202. Kaolin when pure is white and forms China 
clay. It is formed by the decomposition of the feldspars 
and the other silicates by the action of the groundwater 
leaching out the metallic bases, leaving the insoluble sili- 
cate of alumina, which combined with water forms kaolin. 
It is used in the manufacture of china and porcelain ware. 



246 PHYSICAL GEOGRAPHY 

encaustic tile, and as a filler for paper. Mixed with other 
materials it probably forms the bulk of all the clays and 
shales, and a considerable portion of all the soils. 

ORES 

The ores are the minerals from which the metals are 
obtained. A few of the metals such as gold and copper 
occur in the metallic state in nature and are called native 
gold, native copper; but most commonly the metals in 
nature occur combined Avith one or more other elements, 
forming compounds known as ores. The most common 
combinations are with oxygen, forming oxides; sulphur, 
forming sulphides; and carbonic acid, forming carbonates. 

203. Iron, the most useful of all the metals, occurs in 
nature in all three of the above compounds in its different 
ores. 

Hematite (red hematite, fossil ore, specular ore) is at 
present the most important ore of iron in the United States 
and from it more than four-fifths of our iron is manu- 
factured. It occurs in several varieties, some of a bright 
red color, some steel gray, and some almost black. What- 
ever the color of the mass, the streak or powder is always 
red. Hematite consists of ferric oxide, the higher oxide 
of iron. (Fe^O ). The most productive locality for this 
ore is the region about Lake Superior from which much 
of the ore is shipped by boats on the Great Lakes. It is 
mined extensively in Alabama and in smaller quantities 
in New York, Tennessee, Virginia, Missouri, and other 
states. 

Limonite (brown hematite, bog ore, yellow ochre) the 
hydrous ferric oxide (Fe^O^, 2H2O) differs in composition 
from the hematite by having water combined with 
the iron oxide. It has the same composition as the iron 



THE LAND 



247 




Fig. 178. Map of Lake Superior iron ore district and markets. More than 80 
per cent, of the iron ore mined in the United States comes from the Lake 
Superior mines. Much of it is shipped by boat over the Great Lakes. 
The district second in importance is in Alabama. The shaded areas are 
the coal fields. Cities underlined are the principal shipping and receiving 
points. Ore districts at Lake Superior and in Alabama in black. 



248 PHYSICAL GEOGRAPHY 

rust that forms on iron objects exposed to the air. It 
varies in color from yellow ochre to very dark brown, al- 
most black, but the streak is always brown or yellow. It 
forms the yellow and brown coloring matter in nearly all 
the soils and mantle rock. It is deposited in bogs form- 
ing the bog ore, and occurs in many places in the mantle- 
rock, especially in that resulting from decayed limestone. 
It has been mined in hundreds of places along the lime- 
stone areas in the Great Valley of the Appalachians and 
elsewhere. 

Magnetite, the third oxide of iron, consists of the union 
of the ferric and ferrous, or the higher and the lower iron 
oxides, (Fe^O , FeO) and contains a higher percentage 
of iron when pure than any of the other ores. It differs 
from the other oxides and from all other minerals by its 
strong magnetic properties. Three other minerals, one 
variety of hematite and the bronze-colored iron sulphide, 
pyrrhotite, and franklinite are slightly magnetic, but no 
other minerals are attracted as strongly by a magnet as 
magnetite. Magnetite is black in color and the streak is 
black which distinguishes it from the other iron ores. It 
occurs in the Adirondack Mountains, in southeastern New 
York, along the east side of the Appalachians and else- 
where. One of the largest magnetite mines in the world 
is at Cornwall near Lebanon, Pennsylvania. 

Iron pyrites is a yellow, brass-colored mineral, the sul- 
phide of iron, (FeS^) sometimes called ''fool's gold,*' be- 
cause so frequently mistaken for that precious metal. The 
name is appropriate because despite the resemblance in 
color it may be so easily and surely distinguished from 
gold. When placed in the fire or on a hot stove, it turns 
black, gives off the odor of burning sulphur and becomes 
magnetic. It is hard and brittle while gold is soft and 



THE LAND 249 

malleable. While commonly classed with the iron ores, 
pyrites are not used for the manufacture of iron in the 
United States because the sulphur would injure the quality 
of the product. It is used for the sulphur in the manu- 
facture of paper, phosphates, etc. (See fig. 174.) 

Siderite, the carbonate of iron, (FeCOs) is formed by the 
combination of carbonic acid with the oxide of iron. It varies 
in color from gray to brown and sometimes blacl^ as in the black- 
band ore. It occurs associated with coal beds and as black 
nodular masses in the shale beds, where it forms the clay iron- 
stone. It is used extensively in England and formerly in this 
country for the production of iron, but it is used very little in 
the United States at present. 

204. Copper Ores.— In the Lake Superior district 
copper occurs in the metallic state, native copper, but in 
the western areas it occurs mostly in the compounds of 
the metal with carbonic acid or with sulphur. 

CJialcopyrite, the most common copper sulphide, is yel- 
low in color and. is frequently mistaken for iron pyrite. 
It differs from pyrite in being softer, hence more easily 
scratched, having a more golden yellow color and giving 
a blackish green color in the powder. Bornite, another 
sulphide, varies in color, being blue, purple, and yellow. 
There are two carbonates of copper— maZac/ii^e, having a 
bright green color and azurite, a deep blue, both of which 
beside their use as a source of copper are sometimes used 
for ornamental purposes. 

205. Lead Ores.— The most common ore of lead is galena, a 
sulphide of lead (PbS) which looks much like the metal. It crystal- 
lizes in cubes, and has a cubical cleavage, that is, when broken it 
parts along planes parallel to the faces of the cube. Its cleavage 
combined with its brittleness distinguish it readily from the 
metallic lead which it resembles in color. Its color, cleavage, 
and specific gravity (6-7) distinguishes it from other minerals. 
Galena frequently contains silver, and most of the silver mines 



250 PHYSICAL GEOGRAPHY 

produce large quantities of lead as a by-product. Some lead is 
obtained from cerussite, the carbonate of lead. 

206. Zinc Ores.— The chief zinc ore is sphalerite or zinc 
blende, a sulphide of zinc, (ZnS) called "jack" by the miners. It 
has usually a brown color, nearly black at times, and a resinous 
luster. The most productive localities are Missouri, Kansas, 
and Wisconsin. In New Jersey much zinc is obtained from 
franklinite, a bluish-black mineral resembling magnetite, and 
from zincite, a red-colored oxide of zinc. Willemite, a silicate of 
zinc, occurs with the New Jersey ores. 

207. Aluminium Ores.— Bauxite, mined in Alabama and Ar- 
kansas, is practically the only ore of aluminium at the present 
time, although the metal occurs abundantly in many other min- 
erals. Cryolite, formerly used almost entirely as a source of 
aluminium, was at one time "shipped in in large quantities from 
Greenland. Corundum is nearly pure alumina, that is, the oxide 
of the metal; emery, ruby, and sapphire are varieties of corun- 
dum. Aluminium forms a part of the clay in all the great clay 
and shale beds, but it is too difficult to separate from the silica 
in the clay to make the clay a source of the metal. 

208. Amongst other useful minerals are halite, gyp- 
sum, sulphur, graphite, talc, magnesite, fluorite, and apatite. 

Halite, or rock salt is mined from strata deep below 
the surface in New York, Michigan, Ohio, Kansas, Louis- 
iana, and elsewhere. It is frequently obtained by drilling 
wells down to the bed of salt, running in water which dis- 
solves the salt, then pumping out the water and evaporat- 
ing it. At Syracuse, N. Y., the salt is already in solution 
by groundwater, so that it is only necessary to pump out 
the salt water and evaporate it. In places in Utah, 
Nevada, and southern California, salt occurs in great 
abundance on the surface, ready to be gathered up and 
utilized. In some localities it is mined like coal from 
underground workings. In some places it is obtained by 
evaporating the sea water. It is distinguished from all 
other minerals by its taste. (See fig. 179.) 



252 



PHYSICAL GEOGRAPHY 



Gypsum is the sulphate of lime combined with the 
water of crystallization. H=2, luster, pearly to dull. 
Compare with calcite. When heated enough to drive off 
some of the water it forms the plaster of Paris. It is 




Pig. 180. Gypsum quarry, Lyndon, N. Y. The upper 15 feet are limestone. 
The lower part (60 feet) of the quarry consists of gypsum. It is quarried 
for use in the manufacture of Portland cement, wall plaster, and land 
plaster. 



used in making wall-plaster, stucco work, as a fertilizer 
for soil and in the manufacture of Portland cement. 
Alabaster, a variety of gypsum, is used for ornamental 
purposes. Gypsum occurs in beds separated by layers of 
shale and associated with salt beds ii^ many places. It is 
quarried in New York, Michigan, Kansas, Iowa, and many 
of the more western states. (Fig. 180.) 

Sulphur is obtained in Utah, Nevada, California, and Louis- 
iana, but much of that used in the United States is imported 



THE LAND 



253 



from the Island of Sicily. It is used for making matches, gun- 
powder, and sulphuric acid and as a disinfectant. 

Graphite, sometimes called black lead, is a soft, black mineral 
composed of nearly pure carbon. It occurs in the Adirondack 
Mountains, N. Y., and in Pennsylvania, and in several of the 
western states, but the best quality is imported from the island 
of Ceylon. It is used in the manufacture of lead pencils, cruci- 
bles, paint, stove polish and as a lubricant. (Fig. 181.) 




Fig. 181. Interior of Dixon's graphite mine, four miles west of Hague, N. Y. 
The graphite is scattered through the rock which is quarried, crushed, 
• and the graphite separated. (U. S. Geol. Survey.) 



Talc is mined in St. Lawrence county, N. Y., in Virginia, 
Pennsylvania, New Hampshire, and Vermont. It is composed 
of the hydrous silicate of magnesia, is one of the softest (H=l) 
of all the minerals, and has a characteristic greasy or soapy feel. 
It is used as a filler in paper manufacture; the soapstone variety 
is used for switch boards in electrical work, table tops in chem- 
ical laboratories, for household purposes it is used for sinks, 
laundry tubs, cake griddles, foot warmers, etc. Compare speci- 



254 PHYSICAL GEOGRAPHY 

mens of soapstone with foliated talc, describing the differences. 

Magnesite, the carbonate of magnesia, is used in making car- 
bonic acid for soda fountains, as a filler for paper, and for lining 
furnaces. It is quarried to some extent in California but much 
of that used in the United States is imported. 

Phosphates. The phosphate of lime used so extensively as a 
fertilizer consists of the mineral apatite. The purer mineral 
form is quarried in Canada and the more massive rock form is 
quarried in Florida, South Carolina, Tennessee, Alabama and 
elsewhere. 

Fluorite. Fluorite is the mineral formed by the chemical 
union of fluorine and calcium. It crystallizes in cubes and 
octahedrons, but it cleaves more commonly into octahedrons; 
the color is generally green or purple, but it is sometimes color- 
less. It is used as a furnace flux and for the manufacture of 
hydrofluoric acid which etches glass. 

Four of the lime minerals are but one degree apart in the 
scale of hardness and, being common minerals, are usually 
selected as types in the scale: gypsum, 2; calcite, 3; fluorite, 4; 
apatite, 5. 

Besides the minerals mentioned above, there are a hundred 
or more common ones somewhat widely distributed, a number 
of them having some economic importance. There are also 
more than a thousand that are much less common and many of 
them exceedingly rare. 

Make a list of the minerals you have studied with the char- 
acteristic properties of each. 

ROCKS 

The minerals, either singly or in various combinations, 
make up most of the rocks on the exterior of the earth. 
Some rocks, as limestone, or serpentine, are composed of 
a single mineral, while others, as granite or diabase, are 
composed of several different ones. In the glassy vol- 
canic rocks there are no separate minerals, although they 
consist probably of fused minerals which in the glass have 
lost their identity. 

Rocks are commonly grouped in three general classes, 



THE LAND 



255 



based on origin, namely, sedimentary, igneous, and meta- 
morphic. 

209. Sedimentary rocks are formed by the accumula- 
tion of sediments in water, and are therefore stratified. 
Included in this class are certain wind-formed deposits 
that might be distinguished as eolian. The sedimentary 
rocks are divided into the following groups based on the 
chemical composition of the rock mass: 




Fig. 182. Micro-photograph of brown sandstone. The white particles are 
nearly all fragments of quartz. The black part is iron oxide, which gives 
the red or brown color to the rock and acts as a cement to bind the sand 
grains together. 

1. Siliceous. Most of the sand and gravel deposits 
are siliceous and largely, sometimes entirely, composed of 
ground-up fragments of quartz. Along with the quartz 



256 



PHYSICAL GEOGRAPHY 



grains there are frequently variable quantities of frag- 
ments of other common minerals. In sandstones the 
grains are cemented together by some substance, most 




Fig. 183. Brecciated liinef-tom-, Ilighgato Falls Vt. The angular fragments 
of limestone are held together by calcite. A rock in which the fragments 
are rounded is called conglomerate. (U. S. Geol. Survey.) 

commonly clay, iron oxide, calcite, or silica; sometimes 
two or more of these substances may act as cement in the 
same rock. In the process of weathering of sandstones, 
the cementing substance is the first to give way and when 
destroyed the sandstone crumbles to sand, from which it 
was first formed. (See fig. 182.) 

Pebbles or gravel may be cemented in the same way 



THE LAND 



257 



and by the same means as the sand and form conglomerate 
or puddingstone. If instead of the rounded pebbles of 
the conglomerate, the fragments are angular, like broken 
rock, and cemented together, it forms a breccia. 




Fig. 184. Limestone quarry in the Allegaany Mountains at Bellefonte, Pa. 
The limestone layers are nearly vertical due to the folding of the strata 
in the uplift of the mountains. The limestone is here used for making 
quicklime and as a furnace flux in smelting iron ores. In other places 
it is quarried for building stone. One of the best limestones for build- 
ing purposes is quarried extensively in Indiana. 



2. Argillaceous. The argillaceous or clayey rocks in- 
clude the beds of clay or mud as well as the hardened 
forms of these which form the shale beds. Clay may 
grade imperceptibly into shale and this in turn through 
shaly sandstones into sandstones or through calcareous 
shale into limestone and by metamorphism into slate. 

17 



258 



PHYSICAL GEOGRAPHY 



Some of the varieties of clay are china clay, trick clay, 
potters clay, and fire clay. 

3. Calcareous. The calcareous rocks are composed 
of the minerals calcite and dolomite, and include the many 
varieties of limestone and marble. Some of the common 
varieties of limestone are shell limestone; coral limestone; 




Fig. 185. Exposure of a bituminous coal bed near Columbus, Nevada. The 
man's hand is on the top of the coal seam at the contact with the over- 
hanging shale. Coal is a sedimentary rock, and the most important one of 
the fuels. See also Figs. 69 and 231. (U. S. Geol. Survey.) 

chalk; travertine, including the stalactites and the stalag- 
mites of the caves, and the tufa deposits about springs; 
hydraulic limestone or waterlime; marl; and lithographic 
limestone. Gypsum, the sulphate of lime, might also be 
added to the calcareous group. (See figs. 184, 177 and 180.) 

4. Carbonaceous.' The carbonaceous rocks include those 
composed of carbon and the hydrocarbon compounds, as bitu- 
minous and anthracite coal, lignite, peat, asphalt, petroleum, and 
natural gas. (Fig 185.) 



THE LAND 



259 



Ferruginous rocks include the great beds of iron ore. 

Saline rocks include the beds of rock salt. 

Alkaline rocks include the borax and soda deposits occurring 
in arid districts. 

210. Igneous rocks may be divided into the crystal- 
line or granitoid division, sometimes called plutonic; and 




Fig. 186. Granite Quarry, near Barre, Vt. Compare with FiGS. 184 and 185. 
Granite contains joint planes but it is not stratified like limestone and 
coal. (C. H. Richardson.) 

the volcanic or glassy and stony division, to which is some- 
times added a third or intrusive, sometimes called porphy- 
ritic, class. 

The granitoid rocks include those that cool slowly 
under pressure, hence they are crystalline and occur only 
in large masses that were formed deep below the surface 
and are now exposed because of the erosion of the over- 



260 PHYSICAL GEOGRAPHY 

lying rock. They are composed of masses of interlocking 
crystals of different kinds. The granitoid rocks are : 

1. Granite which consists of quartz, orthoclase feld- 
spar, and one, two, or all three of the minerals, mica, 
hornblende, and augite. The micas are more common in 
granite than the other two. (Fig. 186.) 

2. Syenite, which consists of orthoclase and horn- 
blende, augite or mica. It differs from granite in the 
absence of quartz. 

3. Diorite is composed of plagioclase feldspar, and 
hornblende, and thus differs from syenite in the presence 
of plagioclase in place of orthoclase. 

4. Gahhro is composed of plagioclase, augite, and 
commonly magnetite and olivine. It differs from diorite 
in having augite in place of hornblende. It is darker 
colored than diorite, which in turn is generally darker 
than granite and syenite. Diabase is more finely crystal- 
line than gabbro. Basalt which .is still finer grained, 
belongs to the volcanic or intrusive class. The last two 
form most of the trap rock which is used so extensively 
for making good roads. One of the best known exposures 
is in the Palisades on the Hudson. 

The principal volcanic rocks are: (1) olsidian, volcanic 
glass; (2) pumice, rock froth, the very porous material from the 
surface of a volcanic outflow; (3) amygdaloid, the vesicular or 
coarsely porous form with the vesicles, (holes formed by the 
escaping gas), filled with other minerals; (4) trachyte and (5) 
andesite are the two principal stony varieties; (6) porphyry con- 
sists of a fine matrix with imbedded crystals; (7) tufa or tuif is 
composed of fragments such as volcanic ashes or cinders par- 
tially cemented or hardened. 

Pumice is used for grinding and polishing, numbers 4, 5, 6, 
and 7 are used for building stone, and some varieties of porphyry 
form a valuable ornamental stone. 

211. Metamorphic rocks are formed from either sedimentary 



THE LAND 



261 



or igneous rocks by a process known as metamorphism, the chief 
agents of which appear to be water, heat, and pressure. Marble 
is metamorphic limestone and is more crystalline and generally 
harder and brighter colored than the original limestone. Slate 
is metamorphic clay or shale rendered much harder, stronger. 




Fig. 187. Crumpled gneiss, a highly metamorphosed rock. The metamor- 
phism is caused in large part by the lateral pressure which produced the 
wrinkling of the layers. Metamorphic rocks commonly occur in regions 
of folded rocks, that is, in mountains or the eroded remnants of moun- 
tains. (U. S. Geol, Survey.) 



and finely crystalline. Anthracite is thought to be a metamor- 
phic form of bituminous coal produced by pressure and heat. 
Quartzite is a metamorphic sandstone with silicious cement. 
Other metamorphic rocks are gneiss, serpentine, and schists of 
many kinds. (Fig. 187.) 

Selected specimens of the different rocks should be carefully 
studied and compared, and all rocks found in the field trips 
should be named and classified. In many places in the northern 



262 PHYSICAL GEOGRAPHY 

United States specimens of nearly all the rocks described above 
may be obtained from the deposits left by the glacier. 

SOIL 

Soil is produced from the different rocks by a variety 
of processes known as weathering. The principal wea- 
thering agencies are heat and cold, moisture, vegetation, 
wind, and the chemical and mechanical effects produced 
by these. 

212. Disintegration of Rocks.— The heating of the 
rock under the rays of the sun causes it to expand, often 
enough to produce cracks and the breaking off of frag- 
ments. The sudden cooling of the heated rock by a shower 
of rain may produce fracture by contraction. Freezing 
of water held in pores and cavities in the rock breaks and 
fractures it by the expansion of the ice crystals. 

The chemical action of the rain water and the ground 
water breaks up hard minerals, such as feldspar and mica, 
causing them to crumble into clay- and sand. Portions 
of the limestone are dissolved leaving other portions in fine 
fragments in the soil. 

The roots of plants penetrate cavities, cracks, and* 
joints in the rocks and expanding in growing, act like 
wedges to split the rock asunder. Both the living and 
decaying plants furnish organic acids, which, like other 
acids, act on the minerals to dissolve portions of them and 
cause the remainder to crumble to fragments. 

"Winds carry sand and dust against the hard rock, 
grinding off the surface. This agency is most conspicuous 
in dry seasons and in dry climates. 

Gravity acts as a disintegrating agent especially on 
cliffs and steep slopes by pulling down boulders and frag- 
ments, causing a further breaking and crushing in the 
falling, and at the same time exposing fresh surfaces to 



THE LAND 263 

the action of the weather. Gravity carries down large 
quantities of loose mantle rock in the landslide or some- 
times by a very slow process known as creep. (See figs. 
188 and 189.) 

Streams grind away the solid rock along their courses 
and distribute the finely ground-up material over the 




Fig. 188. View near Columbia, Pa., showing creep due to the action of 
gravity on weathered slate. In the upper half of the picture the vertical 
layers are broken and creeping down the hill to the right. (U. S. Gteol. 
Survey. ) 

flood plain, and delta, forming alluvium or alluvial soil. 
The fine sediment carried by the streams into lakes settles 
on the lake bottom and after the disappearance of the lake 
forms lacustrine soil. 

Glaciers in cold climates are active agents in breaking, 



264 



PHYSICAL GEOGRAPHY 



grinding, and disintegrating rock material and transport- 
ing it to other Jocalities. 

213. Mantle Rock.— All of the loose material, fine 
and coarse, that covers the solid bed rock produced by any 




Fig. x89. Creep in glacial clay soil due to gravity. THe ciay when saturated 
with water in a wet season is liable to creep down the hillside, as shown 
in the photo. Sometimes on a steep slope a large mass breaks loose and 
descends rapidly as a landslide. (E. R. Smith.) 



of the above agencies, is known as mantle rock, which 
everywhere rests upon the solid massive rock similar to 
that exposed in the different rock quarries and on the face 
of rock cliffs. 

The change from solid rock to soil is frequently a 
gradual one in which there is no sharp line of separation 
between the two. In fig. 190 the surface is fine soil resting 
upon the subsoil which contains partially disintegrated rock 



THE LAND 



265 



fragments increasing in number and size down to the solid 
rock at the bottom of the quarry. 

The thickness of the mantle rock varies greatly in different 
places. In many places on steep hillsides there is none; in 
other places it is as much as three or four hundred feet deep. 




Fig. 190. Peerless slate quarry, Caml)ria, Md. Showing the change from soil 
at the surface through partially disintegrated rock to the fresh unweathered 
slate at bottom. (Maryland Geological Survey.) 

In the city of Washington the granite rock is disintegrated to a 
depth of more than 80 feet. In places in Brazil, South America, 
the mantle rock is 400 feet deep. Such great thicknesses as 
that are not known in cold climates except where the material 
has been transported as along stream courses in filled valleys, 
alluvial fans, talus slopes, jor deposits made by the glacier. 

Mantle rock changed to soil. The surface portion of 
the mantle rock which has been subject to weathering 
agencies longest, is more minutely divided and broken up 



266 



PHYSICAL GEOGRAPHY 



than the underlying portions. The completely decayed sur- 
face portion which supports plant growth, is called soil and 
the partially disintegrated underlying material is subsoil. 




^*^j*= 



Fig. 191. Weathered surface of granite in Oklahoma. The mantle rock is 
carried away by wind and rain as rapidly as disintegration takes place. 
(U. S. Geol. Survey.) Compare with Figs. 192 and 193. 

The most productive soils contain more or less decaying 
vegetable matter or humus. Probably an important con- 
stituent of productive soils is the bacteria or microscopic 
organisms which hasten the decay of the dead plant and 
the growth of the living one. 

Soils are not very productive until they contain some humus 
or organic material. It may be questioned whether the mantle 
rock is truly soil until it receives this admixture of organic mat- 
ter. This mixing of the organic with the inorganic is accom- 
plished in several ways. The roots of many plants extend to 
considerable depths and leave their substance to decay on the 
death of the plant. Ants, earthworms, and many other burrow- 
ing animals carry vegetable material down and fresh rock mater- 
ial to the surface, thus mixing and fertilizing the soil. The 
farmer assists in this process by the use of the plow and the 
cultivator with which he mixes the organic with the inorganic. 



THE LAND 



267 



214. Varieties of Soil.— The body of nearly all the 
soils is made up of clay and sand. If the sand is absent 
it is clay soil, liable to be cold and wet. The bogs are al- 
ways on clay soils. If the clay is entirely absent the land 
will be sandy, dry, and crops are liable to be burnt out by 
the sun. 




Fig. 192. Hunter ore bank, Center Co., Pa. Residual soil on limestone. 
Part of the soil has been removed to obtain the limonite iron ore which 
is mixed through it. Remnants of the limestone show in the view project- 
ing through the mantle rock. 

There are many grades of soil between pure clay at one ex- 
treme and pure sand on the other. Probably the best soil is 
produced by a somewhat equal mixture of the two called loam or 
loamy soil. The soil in which the sand prevails is best adapted 
to root crops like potatoes and carrots. The clayey soils are 
better suited to grass and similar plants, and are generally 
improved by either surface or tile-draining. 

Along with the clay and sand is a small percentage of the 
elements that give fertility to the soil. The plant derives the 
greater part of its food from the atmosphere, but some of it 
comes from the soil. The nitrates, phosphates, lime, potash, 
and other alkalies, along with organic matter, give fertility to 



268 



PHYSICAL GEOGRAPHY 



the soil by their presence or render it barren by their absence. 
The decaying vegetation and the work of earth worms and other 
animals are important if not essential elements of fertility. 

In many places the soil becomes poor and ceases to produce 
good crops because of the loss of one or more of the above con- 
stituents. The fertility may be renewed by the addition of 
manure or some of the commercial fertilizers. 




Fig. 193. Residual soil resting on marble, Chester, Co., Pa. Part of the soil 
has been removed in order to quarry the marble. The surface of the soil 
is nearly a level plain, but the rock surface underneath is quite irregular. 
The rock surface underneath the soil in Fig. 192 is still more irregular. 

The black lands in many places are composed of a soil that 
is nearly all vegetable material with sometimes a commingling 
of animal remains. This is the mucic which is partially decayed 
vegetation and which forms a soil very rich in the elements of 
fertility, but one lacking in body. 

Residual soil occurs in the place occupied by the orig- 
inal rock from which it was formed. (Fio^s 192 and 193.) 



THE LAND 269 

Transported soils have been carried from the place of 
disintegration by some agency as water, wind, or ice. The 
principal kinds of transported soil are : 

1. Alluvial soils which are among the most produc- 
tive of all soils. There is the happy commingling of sand 
and clay base with the elements of fertility, all of which 
are renewed in the periodic overflow of the stream. 

2. Lacustrine soils which are formed on former lake 
beds and are of three kinds: (a) The black muck such as 
the black lands of New York, formed by vegetable accumu- 
lations generally in small lakes; (b) the fine muds washed 
from the lands and filling in the lake basin. The great 
wheat lands of the northwest belong to the second class, 
(c) Marls formed largely of remains of small shell animals. 

3. Glacial soil. Most of the soil of the northern 
United States is glacial soil and it varies greatly in qual- 
ity. The average productiveness of the whole area has 
been greatly increased by the glacial action, which mixes 
the soil from different localities. In some places, the soil 
is very poor because of too much clay, too much sand, or 
too many boulders, but the average is much better than in 
regions of similar rocks where the soil has not been mixed 
by the glacier. 

215. Life History of Sedimentary Rocks.— The dffferent 
changes which rocks undergo have already been mentioned. It 
remains now simply to connect these stages in the history of 
the rocks to see that the rock material is moving in cycles on 
the earth much as the Water is doing. 

As soon as the rocks are exposed on the surface, the weather- 
ing agencies begin the work of disintegration. Some of the 
materials go into solution and are quickly carried back to the 
sea. Other portions are broken up into fragments large and 
small, which are washed by the rains into streams and carried 
into the sea or lake, where they form beds of gravel, sand, and 
clay. In time these are changed to beds of conglomerate, sand- 



270 PHYSICAL GEOGRAPHY 

stone, and shale or slate, and elevated above the water to be 
again attacked by the weathering agencies and go once more 
through a similar circuit. 

Similarly the limestone beds on the continents are taken into 
solution by the groundwater and through the springs poured 
into the streams and thence carried to the sea where they are 
taken up by the corals, crinoids, shell fish, etc., whose accumu- 
lated remains again form great bedded deposits of limestone 
which are lifted above the sea level to begin a similar cycle over 
again. 

The sand grains and the mud particles in a recently-formed 
rock may have formed part of an indefinite number of similar 
beds of rock in the past. When a bed of rock is weathered, dis- 
integrated, and carried away, the material is only changed in 
positron but not destroyed. The first rock may disappear as a 
rock but the material reappears elsewhere, forming parts of 
other rocks. 

Hardening of the Rocks. The induration or harden- 
ing of the material into solid rock, as the sand to sand- 
stone, lime-mud to limestone, mud-beds to shale, is caused 
partly by the pressure of overlying materials, partly by 
horizontal pressure, and largely by cementing materials 
deposited between the grains by the circulating ground- 
water. The induration may take place before, during, or 
after the elevation, and in some instances never takes 
place at all, as in the case of soft clays that occur in the 
midst of the hard rocks of ancient geological periods. 

216. The Geographic Cycle.— The life history of a 
land area— the geographic cyisle— cycle of erosion— or to- 
pographical cycle, refers to the successive changes in the 
surface features of an area from the time it is elevated 
above sea level until it is worn down to sea level again, 
preparatory to beginning a new cycle. 

Since the development of a river system is at the ex- 
pense of the land area which it drains, the stages of the 



THE LAND 271 

land-area's history are similar to those of the river history 
previously described. 

Youth. Many land areas, when first elevated, have a 
fairly level surface, which is soon made irregular by the 
development and trenching of numerous valleys over the 
area. This incision or deep trenching of the streams dis- 
tinguishes the very early youthful stage of the cycle from 
the final old age stage of the preceding cycle before the 
uplift. The youthful stage is further characterized by 
broad stretches of undrained areas between the streams, 
frequently containing many lakes and swamps. 

Maturity. As the cycle advances from youth towards 
maturity, tributaries to the stream develop in great num- 
bers until all the inter-stream areas are drained. In the 
mature stage (study the Charleston, W. V., sheet) the 
valleys are numerous and deep. Consequently the hill- 
sides are steep and likely to have many bold, rocky cliffs 
and steep talus slopes. The divides consist of narrow 
ridges devoid of lakes and swamps. 

Old Age. The area passes from maturity into old age 
and again approaches a plain in regularity, but a lowland 
plain of old age instead of the upland plain of youth. 
The tops of the hills are lowered by erosion, the talus 
slopes extend to the tops of the hills and spread out 
farther at the base. As the tops of the hills are worn 
down, the level of the valley plain rises, the two approach- 
ing a common level. Finally the harder and more resistant 
rocks which do not erode so rapidly, stand up as knobs 
or prominences, called monadnocks, on the nearly level 
plain-area called a peneplain (pene, almost). (See fig. 194.) 

The erosion continues on the peneplain until the entire 
area is brought to base level, and, in fact, does not wholly cease 
until it is brought to sea level; but the erosion during the pene- 
plain stage proceeds with such comparative slowness, that in 



272 PHYSICAL GEOGRAPHY 

most cases the topography of the peneplain is overtaken by one 
of the movements, either of depression, which causes drowning 
by carrying it below the sea, or of elevation, which causes re- 
juvenation or renewal of youth. 




Fig. 194. Mt. Pony, south of Culpepper, Va. A monadnock on a peneplain. 
The surrounding rocks have been worn down to a lower level than the 
harder, more resistant rocks which form the monadnock mountain. (U. 
S. Geol. Survey.) 

The length of the geographic cycle is determined by a num- 
ber of more or less complex factors, such as: 

(1) The initial elevation, whether great or small, whether it 
is accompanied by metamorphic change or not. Thus an ele- 
vation of metamorphic rocks to great altitudes like the Rocky 
and Sierra Nevada Mountains, will take a much longer period 
of time to be reduced to base level than a low elevation of soft 
material, like the coastal plain of New Jersey or Maryland. 

(2) The vigor of the eroding agents which is determined 
largely by climatic conditions, such as amount arid distribution 
of rainfall and changes of temperature. An area in which the 
rainfall is concentrated in heavy showers will be eroded much 
more rapidly than one in which the rain is more evenly dis- 
tributed throughout the year. 



THE LAND 273 

The annual rain fall in the Bad Lands of Western Nebraska 
is much less than in the fertile districts of Eastern Nebraska. 
Yet the erosion is much greater because the rain is concentrated 
in a few heavy showers separated by long intervals of drouth. 

(3) The resistance offered by the rocks — for example, beds 
of unconsolidated sand and clay may pass through the youthful 
and mature stages to old age in a small fraction of the time re- 
quired by the hard crystalline rocks; in fact, the cycle may be 
completed in the first case before it is scarcely begun in the 
second. 

The length and stage of the cycle are clearly not a question 
of years at all. A pile of soft earth in the laboratory under a 
vigorous shower may be made to pass through all the stages of 
an erosion cycle in a few hours, while an area of hard rocks 
might take many hundreds of thousands of years. 



BEFEBENCES 

Dana, Manual of Mineralogy and Lithology, Wiley & Sons. 

Kemp, A Study of the Rocks, Scientific Publishing Co., New 
York. 

Crosby, Common Minerals and Rocks. 

Howell, Washington School Collection of Rocks and Min- 
erals. 
Soils: 

Shaler, Origin and Nature of Soils, 12th Annual Report 
U. S. Geological Survey, Part 1. Also the last chap- 
ter in "Aspects of the Earth" by the same author. 

Hilgard, Soils and Their Properties, Macmillan Co. 

Hilgard, Relations of Soil to Climate, Bull. No. 3, Weather 
Bureau, U. S. Department of Agriculture. 

Whitney, Some Physical Properties of Soils, Bull. No. 4, 
Weather Bureau, U. S. Department of Agriculture. 

Whitney, Soil Fertility, U. S. Department of Agriculture, 
Farmer's Bulletin No. 257. 

Many of the other publications of the Department of Agri- 
culture contain valuable information on the subjects of soils, 
and would be a great aid to the teacher in this subject. 



18 



CHAPTER VIII 

PHYSIOGRAPHIC AGENCIES 

Diastrophic Movements, Volcanoes, Earthquakes 

Among the most active and important physiographic 
agencies are those of rainfall, weathering, and the work 
of streams and waves described in previous chapters. 
However, one can readily see that the continued action of 
these eroding agencies on the upland areas would in time 
carry all the mountains and plateaus to sea level unles-i 
some new force or forces should work in opposition to ele- 
vate new land areas or re-elevate old ones from time to 
time. Such a force exists in the heated interior of the 
earth and is manifested in volcanoes and in the elevation 
and depression of plains, plateaus, and mountains. The 
force is shown in several ways: there is (1) Diastrophism, 
a very slow movement affecting large areas; (2) Vulcan- 
ism, a rapid outpour of material from the interior of the 
earth to the surface through volcanoes; (3) Seismic move- 
ments, the uplift and depression of large areas by the 
force which produces earthquakes. 

These three phenomena are possibly more or less re- 
lated to each other, yet each may act independently. 

217. Diastrophism '(literally, a twisting or warping) 
is the term used to designate the movements of large por- 
tions of the earth's crust and includes both the movements 
of elevation and depression. An upward movement of 
any portion of the earth 's surface is probably accompanied 
by a considerable depression elsewhere; that is, the sur- 
face is warped or twisted by the action of the internal 

274 



PHYSIOGRAPHIC AGENCIES 275 

forces. Diastrophism, includes both epeirogenic and 
orogenic uplifts. An uplift of a plateau in which there is 
little or no disturbance of the strata is called an epeiro- 
genic movement in contradistinction to an orogenic move- 
ment in which the strata are folded and wrinkled. The 
epeirogenic movement produces plains and plateaus in 
which the strata are horizontal or but slightly inclined. 
The orogenic movement produces mountain ranges in 
which the strata are folded, wrinkled and frequently- 
broken. 

Evidence of elevation and depression. That many of the plains, 
plateaus, and mountains have been elevated to their present 
position by some dynamic force and have not always been at 
this level is proven by the great numbers of fossil-animal and 
plant remains of organisms that live only in the sea, showing 
that these rocks were formed on the sea bottom and then raised 
to their present height. In many places in the rocks there are 
fossil ripple marks that were made in the shallow water of the 
sea. 

On the coasfal plain at Pozzuoli near Naples, the Romans 
erected a temple to Jupiter Serapis. Later the land was de- 
pressed by diastrophism until the building was nearly sub- 
merged in the Mediterranean Sea. This plain was afterwards 
elevated above sea level, and both the elevation and depression 
were so gradual as not to overthrow the temple, three columns 
of which are still standing. They show the effect of their 
former submergence in the sea by the borings of the Lithodomi, 
a species of rock-boring shells that live in the Mediterranean 
Sea. Here is a positive historic example of a depression of a 
coastal plain 25 feet or more and re-elevation of the same, all in 
a period of about 2,000 years. Many other examples, historic 
and geologic, might be cited of both elevation and depression. 
Can the student mention any from his own observation or 
reading? (See fig. 195.) 

218. Cause of Crustal Movements.— The cause of 
the diastrophism is thought to be the shrinkage of the in- 
terior of J:he earth due to the loss of heat. Except from 



276 



PHYSICAL GEOGRAPHY 



changes due to the seasons, the surface rocks have a nearly 
uniform temperature, but the interior of the earth which 
has a much higher temperature is thought to be cooling, 




Fig. 195. Remains of a Roman temple near Pozzuoli, Italy. Since the temple 
was erected about 2000 years ago, the area has been below sea level and 
the three marble columns on the right were perforated by rock-boring 
molluscs. A few feet at the base of the columns were buried in the mud 
and hence not perforated. 

and as it cools it grows smaller. The outer crust, which 
is not losing heat and, consequently, not shrinking, must 
settle down on the decreasing interior portion which causes 
depressions and elevations over the surface. 

Another cause probably acting conjointly with the pre- 
ceding, is the extrusion or transfer of solid, liquid and 
gaseous material and of heat from the interior of the earth 
to the surface through fissures and volcanoes. 

219. Results of the Crustal Movement.— The results 



PHYSIOGRAPHIC AGENCIES 277 

of the crustal warpings are the depressions of great seg- 
ments of the crust, forming the ocean basins, and the ele- 
vation of other portions, forming the great continental 
land masses or portions of the same, the crumpling and 
elevation of mountain chains, the elevation and depres- 
sion of plains and plateaus. 

The elevations may be apparent, not real. That is, 
if the ocean bottom should sink deeper toward the center 
of the earth than the continental masses, the effect would 
be the same as if the land masses were elevated. There 
are some very perplexing problems connected with the 
origin of the continents and ocean basins. 

220. Isostacy.— The principle of isostacy assumes that the 
surface portions of the earth are in temporary equilibrium due 
to equality of gravitative pressure. That is, the continents, 
plateaus, and mountains, stand above sea level because they are 
lighter than rocks beneath the ocean bed which are heavier and 
hence depressed. A disturbance of this equilibrium by mov- 
ing a large quantity of material from one portion of the surface 
to another causes corresponding movements of elevation and 
depression. The erosion of the rocks from the continent and 
the deposition of the material in the margin of the sea causes 
a rising of the continent because of the removal of the land, 
and a sinking of the marginal sea bottom because of the addi- 
tional load. The movement continues until isostatic equilibrium 
is again established or some other force or agency intervenes. 

The great movements which approximately fixed the position 
of the ocean basins and the continental masses, probably took 
place very early in geological history. But since that time, 
changes of level less extensive have been going on from time 
to time which tend to modify the outline and surface features 
of the land areas. 

221. Changes in the Shore Line.— The ocean basins 
are now overflowing, and the overflow laps up over the 
border of the continents on the continental shelf. The 
shore line, or the meeting of the land and sea, may be ex- 



278 



PHYSICAL GEOGRAPHY 



pected to shift from time to time owing to several differ- 
ent causes : 

(1) Primarily the warping or diastrophic movements 
described above. 



Lfff^fS^Wkl^J^^J^mm. I^KT, 




. 



i'lG. 196. General view of Mt. Vesuvius from across the Bay of Naples. City 
of Naples at base of the mountain. Eruption of 1872. Note the great 
volumes of steam from the center of the mountain and from the streams 
of lava on the sides. Torrents of rain descend from the condensing 
vapors. 

y2) The cutting away of the land by the waves on the 
shores and the consequent advance of the sea on the land. 
(Chapter VL) 

(3) Filling-in of the sea bottom by the material car- 
ried from the land by the rivers. 

(4) The infilling from the materials thrown out by 
volcanoes in the sea and from the accumulated organic 
deposits. 



PHYSIOGRAPHIC AGENCIES 



279 



VOLCANOES 

Volcanic eruptions are among the most vivid and im- 
pressive phenomena of nature. The effect produced by 
them on the surface features of the earth are much less 
than those produced by the erosive agencies, yet because 
of the greater intensity of the volcanic forces manifest 
for a short period of time, they make a much stronger im- 
pression on the mind of the observer. 




Fig. 197. Near the summit of Mt. Vesuvius previous to the great eruption 
of 1906, showing the surface of a lava flow in foreground, the great 
ash cone composed of fragments of lava thrown out of the crater, and a 
small cinder cone formed on the surface of a lava stream on the left. This 
part of the mountain was destroyed in the recent eruption. 

222. Mt. Vesuvius.— Mt. Vesuvius, one of the best 
known of all active volcanoes, was considered to be extinct 
at the beginning of the Christian era. In the year 79 
A. D., there was a violent eruption which threw out vast 



280 PHYSICAL GEOGRAPHY 

quantities of fine fragments and water vapor which con- 
densed as rain and fell in torrents. The city of Pompeii 
was deeply buried under the ashes and dust of the erup- 
tion from which it is now being excavated by the Italian 
government. Herculaneum was buried at the same time 
by the ashes and torrents of mud formed by the heavy 
rainfall with the ashes. (See figs. 196, 197 and 198.) 




Fig. 198. Crater of Mt. Vesuvius, a few years previous to the great eruption 
of 1906. Part of a second crater is visible encircling the inner one. 
There was a part of a third, not shown in the picture. 

Following the great eruption of '79, Mt. Vesuvius remained 
quiet for many years, the next outburst occurring in 203, the 
next in 472, again in 512, 993, 1036, 1049, 1138, and 1139, after 
which it remained quiet for nearly 500 years. But during this 
period of quiescence volcanic activity was manifest in the smaller 
volcanoes in the vicinity. 

In 1631 Vesuvius again became violently active, no less than 



PHYSIOGRAPHIC AGENCIES 281 

seven streams of lava flowing out from the crater, partially 
destroying the villages of Ressina, Portici, and Torre del Greece. 
In 1737 there was a stream of lava from the mountain estimated 
to contain 300,000,000 cubic feet. 

Probably one of the most violent eruptions of the mountain 
since 1737 was the recent one in April, 1906, when the vast quan- 
tity of ashes thrown out destroyed the village of Ottajano, while 
great stre.ams of lava extended into and partially destroyed the 
village of Boscotrecasse. Many lives were lost and a great deal 
of property destroyed. 

Besides the many violent eruptions of Mt. Vesuvius since 
'79 and in the preceding ages, there has been a great deal of 
volcanic activity in the region surrounding the mountain, some 
of which is recent and some in times prehistoric. Monte Nuovo, 
a symmetrical volcanic cone, was built upon the coastal plain 
in three days' time in September, 1538. The Solfatara near by 
has been emitting sulphur and arsenic fumes and carbon dioxide 
for centuries. 

On the island of Ischia, there are 12 volcanic cones, one of 
which is now utilized as a harbor for small vessels. The city 
of Sorrento on the other side of the Bay of Naples is built on 
part of a volcanic crater. 

223. Mt. Pelee, Martinique.— There are numerous 
volcanic mountains on several of the West Indies. In 
fact, many of these islands are composed of volcanic rocks, 
but previous to 1902 the volcanoes were thought to be ex- 
tinct. The last eruptions had been in 1718 and 1812 and 
had been forgotten. In 1851 there were earthquakes on 
Martinique and some fine ashes were thrown out of Mt. 
Pelee. 

On April 25, 1902, a great cloud of smoke poured out 
of Mt. Pelee and for several days there were rumbling 
noises accompanied by steam and clouds of dust. On May 
3, there was an eruption which destroyed a sugar factory 
at the base of the mountain and killed a number of peo- 
ple. At 7:50 A. M., May 8, occurred the eruption that 
proved the most destructive to human life of any in 



282 



PHYSICAL GEOGRAPHY 



America. The city of St. Pierre with its population of 
30,000, 17 ships in the bay, and all the country places be- 
tween the E-oxelane and the Riviere Blanche were des- 




Fia. 199. yiew of Mt. Pelee, Martinique, some months after the great erup- 
showing the spine from a distance. Notice the absence of vegetation in the 
vicinity of the mountain. (E. O. Hovey, Am. Mus. Nat. Hist.) 

troyed almost instantaneously by an explosion which shot 
a great volume of hot gas and dust from the top of the 
mountain over the doomed city and harbor. (See figs. 
199 to 202.) 

On July 9th, there was another eruption, thought to 
be similar to the one that destroyed the city of St. Pierre. 
The following graphic account is given by an eye witness* 
of the second eruption: 

"As the darkness deepened, a dull red reflection was Seen 
in the trade wind cloud which covered the mountain summit. 
This became brighter and brighter, and soon we saw red-hot 

*Tempest Anderson in Smithsonian Report, 1902, p. 328. 



OF / 

PHYSIOGRAPHIC AGENCIES 



283 



stones projected from the crater bowling down the mountain 
slopes, and giving off glowing sparks. Suddenly the whole 
cloud was brightly illuminated and the sailors cried, 'The moun- 
tain bursts ! ' In an incredibly short space of time a red-hot 
avalanche swept down to the sea. We could not see the sum- 
mit, owing to the intervening veil of cloud, but the fissure and 




Pig. 200. St. Pierre and Mt. Pelee after the eruption, June, 1902. View- 
looking north. Note the great number of north- south walls, where the 
east- west ones have been destroyed by the blast from the volcano. (Am. 
Mus. Nat. Hist.) 

the lower parts of the mountain were clear, and the glowing 
cataract poured over them right down to the shores of the bay. 
It was dull red, with a billowy surface, reminding one of a 
snow avalanche. In it there were larger stones which stood 
out as streaks of bright red, tumbling down and emitting 
showers of sparks. In a few minutes it was over. A loud. 



284 



PHYSICAL GEOGRAPHY 



angry growl had burst from the mountain when this avalanche 
was launched from the crater. It is difficult to say how long 
an interval elapsed between the time when the great glare 
shown on the summit and the incandescent avalanche reached 





Jb^ 


'^■Ml 


^^^■^ 


-4> 





Fig. 201. Ejected block of lava thrown out of Mt. Pelee Aug. 30, 1902. 
Photographed March, 1903. This block is on the plateau about one mile 
from the crater. (Am. Mus. Nat. Hist.) 



the sea. Possibly it occupied a couple of minutes; it could not 
have been much more. Undoubtedly the velocity was terrific. 
Had any buildings stood in its path they would have been utterly 
wiped out, and no living creature could have survived that blast.'* 
"The most peculiar feature of these eruptions is the ava- 
lanche of incandescent sand and the great black cloud which 
accompanies it. The preliminary stages of the eruption, which 
may occupy a few days or only a few hours, consist of outbursts 
of steam, fine dust, and stones, and the discharge of the crater 
lakes as torrents of water or as mud. In them there is noth- 
ing unusual, but as soon as the throat of the crater is thor- 



PHYSIOGRAPHIC AGENCIES 



285 



oughly cleared and the climax of the eruption is reached, a mass 
of incandescent lava rises and wells over the lip of the crater 




Fig. 202. Near view of the hijge spine protruding from the crater of Mt. 
Pelee. Height about 1200 feet above the rim of the crater. View on 
March 25, 1903. (See Fig. 199) (Am. Mus. Nat. Hist.) 



286 PHYSICAL GEOGRAPHY 

in the form of an avalanche of red-hot dust. It is a lava blown 
to pieces by the expansion of the gases it contains. It rushes 
down the slopes of the hill, carrying with it a terrific blast 
which mows down everything in its path. The mixture of dust 
and gas behaves in many ways like a fluid. The exact chemical 
composition of these gases remains unsettled. They apparently 
consist principally of steam and sulphurous acid. There are 
many reasons which make it unlikely that they contain much 
oxygen, and they do not support respiration." 

A unique feature of the activity of Mt. Pelee was the 
growth in the crater of a monstrous spine or monolith of 
volcanic rock which extended about 1200 feet above the 
rim of the crater (see fig. 202) at its maximum. This great 
spine was an object of absorbing interest and considerable 
speculation on the part of the observers. 

224. Soufriere.— Mt. Soufriere on the island of St. Vincent 
became violently active about the same time as Mt. Pelee and 
on May 7, 1902, a great eruption of this mountain destroyed a 
large amount of property and many lives. 

225. Krakatoa.— In 1883, on the island of Krakatoa, 
in the East Indies, occurred one of the most violent vol- 
canic eruptions known to mankind. For many weeks 
previous there had been great disturbance by earthquakes 
and eruptions of vapor and dust. In such quantities was 
this dust thrown into the air that for 100 miles around 
the island the darkness of midnight prevailed at midday. 

At 10 o'clock Monday morning, Aug. 27, came the cul- 
mination of this disturbance in what was probably the 
loudest noise that has ever been heard on this earth, a 
noise recognized almost 3,000 miles away. Two-thirds of 
the island was blown into the air, some of it to a height of 
17 miles, and some of it pulverized so finely that it was 
three years or more before it all settled out of the atmos- 
phere. Where part of the island stood before the explo- 
sion, the sea was 1000 feet deep afterwards. The enorm- 



PHYSIOGRAPHIC AGENCIES 287 

ous force exerted in such an eruption is almost beyond 
human comprehension. 

226. Definition. — A volcano proper is a pipe-like or 
chimney-like opening in the earth's crust through which 
molten rock, rock fragments, vapor, or gases escape from 
the interior to the surface. Much of the solid material 
so ejected is commonly deposited around the opening, thus 
building up a cone-shaped mountain. The volcanic cone 
has a basin or funnel-shaped depression, the crater, at the 
top which leads into the pipe or neck of the volcano, 
through which the materials are ejected. The cone is not 
an essential part of a volcano, but is generally a product 
of one. (Fig. 203.) 

V 



Fig. 203. Ideal section of volcanic cone, a, Crystalline rocks, b, c, Sedi- 
mentary rocks. V, Crater of volcano, s, Remnant of a former crater. 

227. Volcanic Cones. — The cones are of three types: (1) 
They may be composed of volcanic ashes, cinders, or lapilli, ma- 
terials which have been blown out in a fragmental condition 
and much of which falls around the mouth of the opening, 
building up an ash cone as steep as the fragmental material will 
lie. Mt. Nuovo near Naples is an example. (2) The material 
may come out through the opening quietly and flow away in 
streams or sheets, sometimes a great many miles. The shape 
of the cone in this case depends on the amount, and the tem- 
perature of the material ejected, but generally it has a very 
gentle slope on the exterior, almost flat compared with the ash 
cone. This may be called the lava cone of which the Hawaiian 
volcanoes, Kilauea and Mauna Loa, are type examples. (3) The 
eruptions may vary in the same volcano at different times; at 
one time lava may flow out in streams, and again be blown out 
in fragments; the cone will be built up in part of one and in 
part of the other, forming a mixed lava and ash cone, Vesuvius 
and Etna are examples. 



288 PHYSICAL GEOGKAPHY 

228. Phenomena of a Volcanic Eruption.— The dif- 
ferent kinds of eruptions might be grouped into two classes, 
the explosive and the non-explosive or quiet, with numer- 
ous modifications of each. Both classes are frequently 
but not always preceded by rumblings and earthquakes, 
which continue sometimes for several months before a 




Fig. 204» Driblet cones built of scoria and volcanic bombs on the lava field 
at Cinder Butte, Idaho. Tumuli or small volcanic cones. (See also Fia. 
197.) (U. S. Geol. Survey.) 

great eruption. In the explosive type these disturbances 
are apt to increase in intensity up to the time of explosion. 
The eruption is frequently preceded by the escape of puffs 
of steam and other gases, all terminating finally in a tre- 
mendous explosion which generally destroys the top of 
the volcanic cone. 

In the non-explosive eruption there is a gradual swell- 
ing up and rising of the lava in the crater until it over- 
flows the rim and descends the cone in one or more great 



PHYSIOGRAPHIC AGENCIES 289 

streams. In the great eruption in Iceland in 1783, two of 
these streams flowed down the valley forty-live and fifty 
miles respectively from the source. In the Hawaiian vol- 
canoes, the streams sometimes flow to the edge of the island 
and pour the molten lava into the sea. 

In the very high volcanic mountains the pressure of the 
great vertical column of lava in the crater is sufficient at times 
to burst the cone and form one or more openings in the side, 
through which the lava pours out, building up new cones. 
Sometimes great cracks or fissures form in the side of the cone 
through which the lava flows, and after hardening, forms dikes, 
cutting the sides of the cone. Sometimes on the surface of the 
lava streams or floods there are small cones or craterlets built 
up, through which gas and sometimes lava poured out. These 
are driblet cones. (Fig. 204.) 

229. Materials Ejected.— The materials from a vol- 
cano consist of: (1) gases and vapors; (2) solid materials; 
(3) molten lava. 

The gases consist of water vapor, chlorine, sulphur, 
carbon dioxide, carbon monoxide, arsenic and mercury, 
frequently carrying great clouds of fine dust which ap- 
pears like smoke. 

The solid materials consist of fine fragments called dust 
or ashes ; small pieces, lapilli ; large, irregular masses torn 
from the neck of the opening; and large, rounded masses 
somewhat elongated and pointed, called volcanic bombs. 
The latter are formed by small masses of lava that are 
thrown out while molten, cooling in the passage through 
the air. (Fig. 205.) 

Forms of lava. The hardened lava, free from gas pores, is 
obsidian, which is generally a colored glass resembling cinder 
or slag from an iron furnace. If the lava contains water vapor 
and other gases escaping as it cools, it forms vesicular lava, 
named from the vesicles or bubbles; or if there Is an excess of 

19 



290 



PHYSICAL GEOGRAPHY 



the gas, so that the product is very porous and light, it forms 
pumice. Dark, heavy lava forms basalt on cooling. 

Tufa and dust. The fine fragmental material is sometimes 
cemented by the percolating waters, forming volcanic tufa, a 
soft, porous rock that is used extensively for building stone in 
central and southern Italy, and to some extent in California. 
The volcanic dust is sometimes carried long distances. Large 
deposits of it on the plains of western Nebraska are supposed 




Fig. 205. Elongated volcanic bomb, 13 feet long, on the lava plains at Cinder 
Buttte, Idaho. The bombs are generally shorter than this one. (U. S. 
Geol. Survey.) 



to have blown from the Rocky Mountain area or beyond. Con- 
siderable falls of volcanic dust which have come from some 
distant, but perhaps submarine explosion, are sometimes met 
by vessels at sea. A volcanic eruption is sometimes accom- 
panied by a heavy downpour of rain caused by the condensation 
of the ejected vapors which falling with the dust and ashes, 
forms great streams of volcanic mud, to be changed to tufa as 



PHYSIOGRAPHIC AGENCIES 



291 



the mud dries. It was the downpour of mud that overwhelmed 
Herculaneum, while Pompeii was buried under the volcanic 
dust. 

230. Commercial Products of Vulcanism.— Some of 

the material from volcanoes has economic importance : ( 1 ) 




Fig, 206. Volcanic butte, San Lnis Obispo, Calif. Remnant of a volcanic 
mountain. The porphyritic lava is quarried for building stone. 



extensive sulphur deposits occur in Sicily, Italy, and Ice- 
land; (2) the pumice is used for grinding and polishing 
material; (3) lava and tufa are used for building stone; 
(4) the fine volcanic ash on Mt. Vesuvius and vicinity 
furnishes excellent soil which supports many flourishing 
vineyards; (5) lava is used for road-metal in some places 
and sometimes as ballast for railways; (6) Pozzuolana, a 
fine volcanic ash, is used in making cement in Italy; val- 
uable ores are sometimes found in volcanic rocks. The 



292 



PHYSICAL GEOGRAPHY 




Fig. 207. Muir's Butte, Cal. A volcanic cone on which eroding agencies have 
done little work. Note the symmetry of the cone. (Detroit Publishing 
Co.) 



PHYSIOGRAPHIC AGENCIES 293 

rich gold miines at Cripple Creek and in the San Juan 
Mountains are in volcanic rocks. 

231. Active and Extinct Volcanoes.— Some volcanoes 
are always active, some quiet for years, and some for cen- 
turies. The first are called active, the last are commonly 
classed as extinct, and those in the second class are doj-- 




Fia. 208. Mt. Shasta, California. A volcanic mountain more deeply eroded 
than the preceding. (Detroit Publishing Co.) 

mant. It is not always possible to tell when a volcano 
passes from the dormant to the extinct class. In fact, some 
that were thought to be extinct have become active, Mt. 
Vesuvius was supposed to be extinct previous to the great 
eruption in the year 79 which destroyed Herculaneum and 
Pompeii. Mt. Pelee had not been active since 1851 until 
the great eruption in 1902. 

There are 300 active volcanoes, that is, 300 have been 
active during recent years. How many of the so-called 
extinct and dormant ones may become active at any time 
is not known. 



294 



PHYSICAL GEOGRAPHY 



232. Effect of Erosion on Volcanoes.— The eroding agencies 
are always at work on the volcanic cones, even while they are 
erupting, but each eruption generally obscures the effects of 
previous erosion. After activity ceases the effects of erosion 
are soon manifest. The cone is dissected by streams and car- 
ried away through radiating valleys much like any other moun- 




FiG. 209. Devil's Tower, Wyoming. Remnant of an eroded volcanic moun- 
tain. By some this is thought to be the remnant of a laccolite (Fig. 211) ; 
others consider it the throat of a volcano from which the surrounding ash 
cone has been eroded. (U. S. Geol. Survey.) 



tain peak. (Mt. Shasta should be studied as a type of volcanic 
cone in youthful stage of erosion. See Folio 2, Top. Atlas, U. S. 
Geol. Surv., study figs. 207, 208, and 209.) 

Sometimes the cone is composed largely of ashes and the 
throat-opening is filled with rock cooled from a molten state. 



PHYSIOGRAPHIC AGENCIES 295 

In such a case the softer exterior will be eroded rapidly, and 
leave the harder, more resistant rocks of the neck or plug stand- 
ing as a prominent elevation. (See fig. 209.) 

233. Calderas. — Sometimes the bottom of the crater 
subsides as activity ceases, leaving a large and often deep 
depression, called a caldera, which when filled or partly 
filled with water forms a crater lake. Crater Lake near 
Mt. Hood in Oregon is an example. (Study Crater Lake 
topographic sheet and explanation in Folio 2 of Top 
Atlas.) (See figs. 75 and 76.) 

Part of the calderas, at least, are thought by some to 
be formed by the bottom, and part of the rim being blown 
away by a violent explosion instead of by subsiding. 
Sometimes an eruption terminates the existence of a crater 
lake, as was the case on Mt. Pelee where the small lake 
that was there previous to the eruption was entirely 
destroj^ed. 

234. Distribution of Volcanoes.— The greater num- 
ber of active volcanoes are located around the border of 
the Pacific Ocean. There is a chain of them extending 
from Cape Horn at the extremity of South America along 
the western border of both Americas, across the Aleutian 
Islands to Asia, and down the Asiatic coast. There are 
several groups in the Pacific Ocean, a number in the At- 
lantic Ocean, some on Iceland and the West Indies, and 
another great group in the Mediterranean Sea around the 
south end of Italy. 

There is good reason for thinking that there are many vol- 
canic peaks scattered over the sea bottom. The Hawaiian Is- 
lands and many other islands are but the tops of volcanic moun- 
tains, built up on the sea bottom, while no doubt there are many 
others whose tops are below the surface of the sea. 

In 1867, among the Tonga Islands in the Pacific, a shoal 
was discovered surrounded by water 6,000 feet deep. Ten years 
later steam was observed rising from this shoal and in eight 



296 PHYSICAL GEOGRAPHY 

years more there was an island of volcanic ashes two miles 
long and 200 feet high. Unless there is further activity the 
island will soon be cut away by the waves and again form a 
shoal. 

During the time of, or immediately following the San Fran- 
cisco earthquake, a new volcanic island appeared in the Bogoslof 
group among the Aleutian Islands. 

235. Life History of a Volcano.- The life history of 
a volcano begins with some changes deep below the surface 
and beyond our observation. Although the outbreak is 
frequently preceded by earthquakes, the first visible evi- 
dence is a crack or opening extending downward indefinitely 
through the solid rocks. Through this opening are ejected 
gases, molten lava, and heated rocks, part of which accu- 
mulate around the top of the opening, building up in time 
a cone-shaped mountain. This period of up-building may 
be called the period of youth and growth. The volcano in 
its maturity is a lofty mountain peak which finally ceases 
to erupt and becomes extinct. The eroding agencies which 
have been overshadowed in this upbuilding process now 
show the effect of their activity in wearing away the top 
and sides of the mountain. The first step is to carry away 
the soft fragmental, cinder portion of the cone, leaving the 
harder central core or neck which is finally worn down to, 
or below, the level of the original area on which it started. 

236. Fissure Eruptions.— Sometimes eruptions take place 
through elongated openings, or fissures, instead of through 
chimney-like rpenings or craters, and are then called fissure 
eruptions. They do not build craters, but spread out over the 
adjoining region in great sheets or floods, sometimes many 
hundred feet thick. Such are the great lava fields over parts 
of Washington, Oregon, Idaho and California, covering an area 
of 200,000 square miles, thousands of feet in thickness. 

Ther is no sharp separation between eruptions through 
craters and through fissures. In fact, small fissure eruptions 
occur frequently on volcanic cones where the pressure from the 
interior frequently forms cracks or fissures in the side or base 



PHYSIOGRAPHIC AGENCIES 297 

of the cone, through which the lava may flow to the surface in- 
stead of overflowing the rim of the crater. The hardening of 
the material in the fissure forms a dik?. 

Dikes of igneous rocks may form likewise in places remote 
from any volcanic cone, such as those in the city of Syracuse, 




Fig. 210, Devil's Slide, Colorado. A dike of igneous rock. The 
central portion disintegrated more rapidly, causing the de- 
pression. The outer portions of the dike are more durable 
causing them to stand up as walls above the surface. 

N. Y., and in the vicinity of Little Falls, Ithaca, and elsewhere 
in New York State, in many places along the Appalachian 
region, also around Lake Superior and in many other localities. 
The Palisades on the Hudson are composed of igneous rocks 
that came up in a molten state through great fissures or were 
forced out between layers of other rocks. (Fig. 210.) 

237. Laccolites.— Sometimes the molten material rising 
through fissures does not reach the surface but pushes up the 
overlying rock and spreads out between the layers in a mushroom- 



298 



PHYSICAL GEOGRAPHY 



shaped mass called a laccolite. (See fig. 211.) It is called a 
sheet or sill if it spreads out between the strata in flat sheets 
without arching the overlying rock. 

238. Causes of Vulcanism.— There are some features 
connected with volcanic eruptions that are not well under- 





FiG. 211. Laccolites in Henry Mountains, Utah. Upper figure shows section 
through laccolite as first formed. Lower figure shows by dotted lines part 
removed by erosion. (After Gilbert). 

stood. The source of the heat that melts the rock is not 
known certainly; it may in part be produced by pressure 
and gravity, in part by chemical action, in part by residual 
heat of the earth. The force that lifts the huge column of 
molten rock 10,000 feet or more above the level of the sea 
may be due to expansion of the rocks by heat, and espe- 
cially the expansion of the gases and vapors included in 



PHYSIOGRAPHIC AGENCIES 299 

them, aided by the gravitative downward pressure of sur- 
rounding heavier material. The cause of violent explo- 
sions that blow out such vast quantities of fragmental 
material is probably the expansion of the gases, especially 
water vapor. 

EARTHQUAKES 

Earthquakes are important geographic factors in their 
effect on topography, on life, and in their relation to vol- 
canoes. There have been at least three destructive earth- 
quakes in the United States during the past century be- 
sides hundreds of minor ones, of which there is little or no 
record. 

239. Mississippi Valley Earthquake, 1811.— The first of the 
great earthquakes in this country, of which there is any written 
account, began in the Mississippi Valley between St. Louis and 
Memphis on December 16, 1811, and continued at intervals for 
several months. It began at two o'clock in the morning, when 
the people awoke to find chimneys falling, furniture thrown 
about and the earth rocking and trembling. 

"At 7 o'clock a rumbling like distant thunder was heard and 
in an in *ant the earth was convulsed so that no one could 
stand. Looking at the ground the terrified people saw it rise 
and fall, as earth waves like those upon the sea rushed past, 
waving the trees until the branches interlocked, and causin,g 
yawning cracks to open. Giant forest trees were split for 40 
feet up, half standing on one side of the fissure, the remainder 
on the other. Some of the earthquake rents were of great 
size, having widths of 30 feet or more, while some are reported 
as many as five miles in length. Others were circular in form. 
Into some of the cracks rushed the waters from swamps and 
bayous, while elsewhere small streams or even rivers left their 
old beds and made new channels through the cracks. 

"In some places there was a blowing out of the earth, bring- 
ing up coal, wood, sand, etc., trees being blown up, cracked and 
split, and falling by thousands at a time. 

"Many of these great fissures are still open at the surface, 
and steep banks formed by landslips are still visible. Several 



300 



PHYSICAL GEOGRAPHY 



lakes were formed on river bottoms. Reelfoot Lake, in western 
Tennessee, formed at this time, is five miles wide, twenty-five 
miles long and twenty-five feet deep. 




Fig. 212. EartlKiuake fissure 8 to 10 feet deep on the hill east of Reelfoot 
Lake, Teun., formed m 1811. (M. L. Fuller.) 



"During the three months following December 16, there 
were recorded in the Mississippi Valley 1,874 earthquake 
shocks, of which eight were violent, ten severe, and thirty-five 
alarming."* (See figs. 212 and 77.) 

240. Charleston Earthquake.- On August 31, 1886, 
the city of Charleston, S. C., and the surrounding region 
were severely shaken by an earthquake, the effects of which 
were felt as far north as New England and as far west as 
Minnesota. Nearly every building in the city of Charles- 
ton was injured to some extent, many of them severely in- 

*M. L. Fuller, Pop. Sci. Mon., July, 1906, 



PHYSIOGRAPHIC AGENCIES 



301 




(50SEI5MAL3 OF THE 

Ca\RLl bTO\ £.\R1HQUAKE 

ROSSI FORCLSCAL£ 
ScWkoTMUM 



Fig. 213. Map showing area of disturbance by the Charleston Earthquake, 
1886. The concentric lines are the Isoseismals which converge around the 
centre, near Charleston. The intensity was greatest near Charleston, and 
least in Vermont and Minnesota, beyond which it was not perceived. 
(U. S. Geol. Survey.) 



302 



PHYSICAL GEOGRAPHY 



jured and some totally demolished. There was consider- 
able loss of property and life in the area surrounding 
Charleston. Fissures were opened in the earth, springs 
ceased flowing in some places, and broke out in other places. 




Fig. 214. San Francisco earthquake, April, 1906. Earthquake fracture near 
Olema, California. The part on the left of the bank has been thrust away 
from the observer. The fence was built after the earthquake. (J. F. New- 
som.) 

railway tracks were twisted and distorted and in one in- 
stance a locomotive was thrown from the track. 

241. San Francisco Earthquake.— Every year there 
are many earthquakes in California, but most of them are 
not perceptible to the senses and rarely are any of them 
very destructive. In 1872 an earthquake destroyed the 
village of Lone Pine and formed a great fissure through 
the Sierra Mountains for 200 miles. 



PHYSIOGRAPHIC AGENCIES 



303 



The earthquake which destroyed more lives and prop- 
erty in California than any other was the one that took 
place in the early morning of April 18, 1906. There were 
a number of minor shocks at intervals for several days, 
but nearly all the damage was done in 65 seconds time. 




Tig. 215. San Francisco earthquake. On the railway between Los Gatos and 
Santa Cruz. The rails were stretched so much that a piece had to be cut 
out before the road could be straightened. (J. C. Branner.) 



A large part of the city of San Francisco was destroyed 
by the earthquake and the fires which were caused by it. 
Great damage was done at Santa Rosa, San Jose, Stanford 
University, and many other towns in the area of destruc- 
tion. A great many buildings were thrown down or other- 
wise wrecked. Nearly all chimneys were shaken down. 
Water pipes, sewers and bridges were rent apart. Trees 
were uprooted in large numbers, some were snapped off, 
leaving the stumps standing. Fissures were opened in 



304 



PHYSICAL GEOGRAPHY 



the earth and closed again ; in one place it was reported 
that a cow was engulfed. Line fences were moved. Roads 
and railways were twisted and shifted. In one place the 
steel rails were stretched so that it was necessary to cut out 
a piece several inches long before they could be replaced. 
(See fig. 215.) 




Fia. 216. San Francisco earthquake, A vertical drop of seven feet east of 
Watsonville, California. The foreground was on a level with the orchard 
before the earthquake. Note the craterlets in the foreground near the 
break. (J. C. Branner.) 

The earthquake was caused by a great fissure or crack 
extending 375 miles from Point Arena on the north to 
Mount Pinos on the south, along which the earth was frac- 
tured and shifted horizontally a distance varying in dif- 
ferent places from six to twenty feet. The area of destruc- 
tion following this line extended over 400 miles long and 
fifty miles wide, but the shock was felt as far north as 



PHYSIOGRAPHIC AGENCIES 



305 



Coos Bay, Oregon, and eastward into Nevada. The passage 
of the earthquake was recorded by delicate instruments as 
far away as Sitka, Alaska ; Washington, D. C. ; Tokio, 
Japan, and Potsdam, Germany. It probably passed around 
the globe ^(Figs. 214 to 217.) 




Fiu. 217. San Francisco earthquake. View on a road across alluvial lands 
near Salinas, California. The stair-step appearance is due to the breaking 
and sinking down along the fractures. This is not on the main fracture or 
fault plane which produced the earthquake. (J. C. Branner.) 

This earthquake was probably no more violent than the 
one mentioned in 1872 and probably others, and much less 
violent than the one in 1811, but the fact that it occurred 
in the most densely populated part of California made the 
destruction of human life, and property much greater than 
any other recorded in the United States. 

242. Kingston Earthquake.— There have been a great many- 
destructive earthquakes in Central and South America and the 

20 



306 PHYSICAL GEOGRAPHY 

West Indies. One of the most disastrous in recent times was 
that which destroyed the city of Kingston, Jamaica. The first 
shock came at 3:30 P. M., January 14, 1907, and was followed by 
fifteen severe shocks during the following week. More than one 
thousand persons were Rilled and nearly every building in the 
city was injured; many of them were totally destroyed. 

The area affected by the Kingston earthquake was much 
smaller than the others mentioned. There seems to have been 
little damage done outside of a radius of ten miles from the city. 
The old city of Port Royal, just across the bay, was severely in- 
jured during the earthquake in 1682, when part of the city sank 
beneath the ocean. The remnant of the old city was injured in 
the recent quake. 

243. Earthquake of Lisbon.— One of the most destructive 
earthquakes recorded in human history, was that on November 
1, 1755, when the city of Lisbon was destroyed and over 60,000 
persons perished in a few minutes. It began with noise like 
heavy thunder which was immediately followed by a most violent 
agitation of the surface, in which the ground rose and fell like 
the waves of the sea; the neighboring mountains were shaken 
like reeds and rent asunder in many places. Great chasms 
opened in the city into which large buildings tumbled and dis- 
appeared from view. The waters of the ocean retreated at first 
and then returned in a great sea wave, fifty feet high, which 
rushed over the doomed city, completing the ruin caused by the 
shaking. The area of destruction extended as far away as the 
Alps Mountains and across into northern Africa where several 
villages were destroyed. 

Japan is now the foremost nation in the scientific investiga- 
tion of earthquake phenomena. It was aroused to action by the 
severe earthquake of October, 1891, in which 7,000 people were 
killed, 17,000 injured, and 20,000 buildings destroyed. 

244. The Cause of Earthquakes.— Anything that pro- 
duces a violent jar or concussion in the earth will produce 
an earthquake. The explosion of a heavy blast in a mine 
causes a trembling of the earth. The explosion of the 
chemical works near San Francisco some years ago was 
felt forty miles away. In nature the following agencies 
produce earthquake waves: (1) A sudden fracturing of 



PHYSIOGRAPHIC AGENCIES 307 

the rocks or the slipping or shifting of a large mass of 
rock along a fissure or fracture in the earth's crust pro- 
ducing a geological fault; (2) the explosion of a large 
volume of gas or steam; (3) the slumping off of a large 
mass of sediment from the edge of the continental shelf 
into the deep ocean basin. 

245. Distribution of Earthquakes.— Earthquakes are 
common in volcanic regions, and in mountainous countries, 
especially in young and growing mountains. They are 
numerous and destructive around the Pacific Ocean, fol- 
lowing the volcanic zone; but they are not limifed to vol- 
canic regions as shown by the fact that the most severe 
earthquake recorded in the United States took place in the 
Mississippi Valley as remote from mountains and volcanoes 
as is possible in this country. 

Earthquakes may occur anywhere at any time, but they 
are not likely to be either severe or abundant in eastern 
or central United States in comparison with the Pacific 
border. 



BEFEBENGES 

Volcanoes : 

Russell, Volcanoes of North America, Macmillan & Co., 1897, 
$4. 

Hull, Volcanoes Past and Present, Scribner's Sons, N. Y., 
1892, $1.50. 

Judd, Volcanoes, D. Appleton & Co., N. Y., 1881, $2. 

Bonney, Volcanoes, Putnam's Sons, N. Y., 1899, $2. 

Geikie, Ancient Volcanoes of Great Britain, The Macmillan 
Co., N. Y., 1897, $11.25. 

Diller, Mt. Shasta, National Geographic Monographs, Amer- 
ican Book Co., 1895. 

Dana, Characteristics of Volcanoes, Dodd, Mead & Co., N. Y., 
1891, $5. 

Button, Hawaiian Volcanoes, 4th An. Rept. U. S. Geol. Sur- 
vey, p. 8. 



308 PHYSICAL GEOGRAPHY 

Heilprin, Mt. Pelee and the Tragedy of Martinique, Lippin- 

cott, Phila., 1903, $3. 
Phillips, Vesuvius, The Macmillan Ck)., 1869. 
Lobley, Mount Vesuvius, London, 1889. 
Hovey, Eruptions of 1902, of La Soufriere, St. Vincent and 

Mt. Pelee, Amer. Jour. Sci., Nov., 1902, vol. 14, p. 319. 
Earthquakes : 

Arland, Great Earthquakes, New York, 1887. 

Button, Earthquakes in the Light of the New Seismology, 

London, 1904. 
Charleston Earthquake, 9th An. Rept. U. S. Geol. Survey. 
Milne, World-Shaking Earthquakes, Scottish Geog. Mag., Oct., 

1902. 
Perrine, Earthquakes in California, Bull. 161, U. S. Geol. Surv. 
San Francisco Earthquake, National Geog. Mag., May, 1906, 

Pop. Sci. Mo., Aug., 1906. 



CHAPTER IX 
PHYSIOGRAPHIC FEATURES OR FORMS OF RELIEF 

Plains, Plateaus, Mountains 

If one should travel westward across the United States 
from Atlantic City on the eastern coast to San Francisco 
on the western, he would traverse a succession of plains, 
plateaus and mountains. The same would be true on any 
other continent, but the order, relative size and features 
of detail would be different. The surface of all the con- 
tinents and islands consists of these three natural features 
with endless variety in each. 

The line of separation between mountains and plateaus 
or between plateaus and plains is not always sharply 
marked. In travelling west across the Appalachian plateau 
it would be difficult for any one to locate the exact spot 
where he passes from the plateau to the Mississippi plains. 

A desert is a specified form of one of the other physio- 
graphic features, a form based on a difference in climate. 
It may be part or all of a plain or plateau and may contain 
mountains. 

PLAINS 



i 



246. Plains arej areas of low relief, comparatively 
smooth surfaces, generally at no great elevation above sea 
level. However, the Great Western Plains rise towards 
the west to heights several thousand feet above that of 
some plateaus. Why are they called plains instead of 
plateaus ? 

The greater part of the food of the world is raised on 

309 



310 



PHYSICAL GEOGRAPHY 



plains, and for that and other reasons (what other reasons 
can you give?) the greater part of the population of the 
world lives on the plains. They are therefore important 
geographic features from an economic standpoint. 




Pig. 218. Banded coastal plain. The strata dip towards the sea. The hard 
layers form ridges with steep face towards the interior. The consequent 
streams flow towards the sea. The subsequent streams develop along the 
edges of the soft layers at right angles to the first streams. Conditions 
are favorable for artesian wells. 



A coastal plain is an uplifted portion of the continental 
shelf which has been added to the former land area as the 
shore line receded seaward. A coastal plain may be nar- 
row and composed of one kind of material, resulting in a 
simple consequent drainage in which the main streams 



PHYSIOGRAPHIC FEATURES 311 

flow in the general direction of the slope of the land ; or it 
may be broad and composed of successive layers of differ- 
ent kinds of rocks which, if they differ in hardness, result 
in parallel ridges, and belts of different kinds of soil par- 
allel to the shore. Such a plain may be distinguished from 
the preceding by calling it a helted or handed coastal plain. 
Study carefully fig. 218 and explain the significance of the 
name. 

If some of the layers are more porous than others, as 
is generally the case, the conditions are favorable for 
artesian wells. Long Island, Southeastern New Jersey, 
and Eastern Maryland are portions of a belted coastal 
plain composed of beds of sand and clay dipping towards 
and underneath the sea. The rain that falls on the out- 
cropping edges of the sand layers is carried down as 
groundwater underneath the overlying clay beds which 
prevent its escape to the surface, and hence causes accumu- 
lation under pressure and consequent rise of the water in 
the artesian-well opening. Artesian wells may be sunk 
on the extension of such a plain under the shallow sea as 
well. as on the land plain. (See figs. 219 and 35.) 

An ewhayed coastal plain is one which, after elevation and 
erosion, has been depressed, causing the shore line to again ad- 
vance on the land. The sea will now advance far up the river 
valleys forming bays and estuaries, thus drowning the lower val~ 
leys and dismembering many of the streams. In the case of the 
belted plains, there may be one or more lagoons parallel with the 
shore over the eroded surface of the softer layers which had 
been worn down nearly to sea level before the depression. The 
higher, harder rocks then form peninsulas or islands. 

There are many ancient coastal plains now in the interior of 
the continents and remote from the sea, because since their first 
uplift there have been successive additions to the plain. In 
some instances ev^en mountain ranges have been elevated be- 
tween the old coastal plain and the present sea shore. 

Most of the ancient coastal plains that are now far inland 



312 



PHYSICAL GEOGRAPHY 




PHYSIOGRAPHIC FEATURES 313 

are more diversified than the recent ones. The rocks are gener- 
ally harder and more resistant and frequently the surface is 
more rugged. 

The greater part of New York State has been in times past 
part of a coastal plain. The sea surrounded the Adirondack 
Mountains and by successive uplifts of the land the shore-line 
retreated south and west, exposing additional strips of coastal 
plain. Study a geological- map of New York. 

Economic features of coastal plains. Coastal plains are 
generally covered with a good soil, they have a good water 
supply, and commonly a fairly regular surface, all of 
which favor an extensive agricultural industry. Road 
construction is for the most part easier than in the hill 
country, which fact favors both agriculture and commerce. 
Sometimes the commerce by water is hampered by the lack 
of good harbor facilities, but generally there is consider- 
able water traffic both in the coast trade and on the rivers 
which flow across the plain. They also favor manufactur- 
ing industries where there are good harbors or navigable 
streams, because of the favorable position for distributing 
the products of the factory. 

247. Alluvial Plains.— Flood plains occur along the 
older portions of nearly all rivers. As soon as the river 
has cut down to grade, it begins cutting at the cliffs, some- 
times on one side and sometimes on the other. Part of 
this material and part of that brought from the head 
waters is spread out over the floor of the valley, building 
up a flood plain over which the river takes a meandering 
course. During high water when the river overflows its 
banks, sediment is deposited over all the area flooded, 
but in greater quantities along the immediate banks of the 
stream, building up embankments or natural levees, which 
grow higher from flood to flood until finally the river 
breaks through and takes a new course on the lower part 



314 



PHYSICAL GEOGRAPHY 



of the plain which in turn is built up in a similar manner. 
(See figs. 220, 221, 54, 55, 58 and 67.) 

One effect of the upbuilding of the natural levee is to cause 
the surface of the plain to slope away from rather than towards 




Fxu. 220. View on the flood plain of the Bittcnoot river in Montana. The 
alluvial soil is very productive. (U. S. Geol. Survey.) 



the river channel, so much so that in some places small streams 
develop on the levee bank and flow directly away from the chan- 
nel into the river swamp. (See Donaldsonville topographic sheet.) 
Furthermore, the levee forms an obstruction to the junction of a 
tributary with the main stream. On the lower Mississippi river 
flood plain, some of the tributaries flow along the outer margin 
of it for many miles before they flnd an opening through the 
levee into the main river. (See fig. 221.) 

On the delta, where there are no bordering bluffs, portions 
of the main stream break through or overflow the levee and flow 
directly to the sea as distributaries. 

The delta plain forms the continuation seaward of the river 



PHYSIOGRAPHIC FEATURES 



315 



flood plain. The delta plain of the Nile has for many centuries 
been one of the most populous portions of the globe. 

The greater part 
of a flood plain is 
fertile land, well 
adapted to agricul- 
ture because: (1) It 
is covered with rich 
humus carried from 
the uplands and hill- 
sides; (2) the fertil- 
ity is renewed from 
time to time by the 
overflows, which de- 
posit a new layer of 
fertile soil; (3) the 
water-table lies so 
near the surface that 
the area does not suf- 
fer from drouth so 
much as the upland; 
and (4) transporta- 
tion and communica- 
tion, save in the flood 
season, are easy by 
road, railway and ^ 
river. For these rea- 
sons a large part of 
the food products of 
the world is raised 
on alluvial river 
plains and deltas. 




Fig. 221. Map of a portion of the lower Miss- 
issippi River flood plain showing effect on 
the tributaries of the up-building of the flood 
plain along the channel. Note how many of 
the tributaries run nearly parallel with the 
river for long distances. The dark lines along 
the river are artificial levees. (U. S. Geol. 
Survey. ) 



The greatest drawbacks to prosperity on the flood plains 
are (1) destruction of life and property from the floods, 



316 PHYSICAL GEOGRAPHY 

and (2) malarial climate from the many mosquitoes which 
breed in the stagnant water. Many attempts have been 
made to remedy the danger from floods by building higher 
embankments on the levees to keep the water in the chan- 
nel. But the exceptionally high flood finally overflows 
and causes injury to both life and property. In a dry 




Fig. 222. Brazos Island, opposite Point Isabel, near Brownsville, Texas. 
The effect of grass in holding the drifting sand. River flood plain in an 
arid region. (W. L. Bray.) 

climate, a shallow river sometimes deposits much sand in 
and along the channel. Strong winds spread the sand 
over the adjoining fertile areas. (See fig. 222.) 

248. Lacustrine Plains.— A lacustrine plain is one 
that was formerly covered with a lake. The plain may 
represent the entire area of the former lake or only a por- 
tion of it and may be produced by: (1) the filling of the 
lake; (2) the draining of it by the cutting down of the 



PHYSIOGRAPHIC FEATURES 317 

outlet; (3) the evaporation of the water by a change in 
climate; (4) diastrophism elevating a portion or all of 
the lake bottom; or sometimes a combination of two or 
more of the above ways. 

The famous celery and onion black land areas over the north- 
ern United States are almost all lake plains formed by the filling 
partially or completely of small lake basins with vegetable and 
animal matter. The numerous vlies or meadows of the Adiron- 
dacks are similarly formed. 

The great wheat lands of the northwest, in Dakota, Montana 
and Canada are lake plains on the bottom of large extinct lakes. 
Lacustrine plains are almost universally fertile, since they con- 
tain so much organic matter and the rock material is so very 
finely disintegrated. 

In places around the margin of extinct or fossil lakes, there 
are sand and gravel deposits corresponding to the sand and 
gravel beaches now forming in many places along the shores of 
the larger lakes. These are lacking in fertility and have little 
value for agriculture, but in the vicinity of cities they furnish 
sand and gravel for building purposes. Nearly all the sand and 
gravel used in Syracuse, New York, are obtained from the old 
beaches of an extinct lake on the bed of which a considerable 
portion of the city is built. 

249. Salt and Alkali Plains.— Lacustrine plains in arid 
regions are sometimes covered in part at least with salt or 
alkalies, sometimes in such quantities that they can be 
shovelled into cars and shipped to market. Shipping salt 
in this way was an important industry at Salton, Cali- 
fornia, before the Colorado River broke loose and covered 
the area with water. Travel over the extensive alkali plains 
west of Great Salt Lake in Utah and Nevada is disagree- 
able in dry weather because of the alkali dust. 

250. Glacial Plains. — A continental glacier, such as 
that which covered the northern United States in former 
times, leaves many plains, large and small, on the area 
which it covered. Probably the glacier does not produce 



318 PHYSICAL GEOGRAPHY 

many plains, but it modifies the surface of those already in 
existence, possibly increasing the size of some of them. 

The glacial plains may be covered with glacial till or boulder- 
clay or with sand and gravel as in the overwash plains and 
glacial aprons. In some places they are partly covered with 
large boulders. Many of the lacustrine plains of the northern 
United States are indirectly glacial, as it was the glacier that 
formed the lake basin and sometimes helped to fill it. 

251. Peneplains.— Some plains are formed by exten- 
sive erosion of a former plateau or mountain area. As the 
rivers with their tributaries cut the upland plateau or 
mountain down nearly to base level, the divides between 
the valleys are finally lowered to near the valley levels 
until the entire area approaches a plain surface which is 
called a peneplain, meaning almost a plain. The harder, 
more resistant rocks on the area remain as hills or eleva- 
tions on the peneplain, and the more prominent ones are 
known as monadnocks. 

Nearly all of New England was at one time reduced by ero- 
sion to a peneplain, which was then elevated. The streams 
deepened and widened their valleys until the uplifted plain or 
plateau was much dissected. The upland areas between the 
streams are all that is left of the former peneplain. (See "The 
New England Plateau" by W. M. Davis, one of the National 
Geographic Monographs.) 

Around the base of high mountains there are fre- 
quently plains, large and small, that have been built up by 
the material carried down the mountain and spread on the 
low ground at the base. They are conspicuous topographic 
features in the arid and semi-arid mountainous regions of 
the west and southwest United States. Fig. 223 is a view 
on one of these intra-montane (among the mountains) 
plains near Acton in southern California. Such areas 
have been called plains of aerial aggradation. 



320 PHYSICAL GEOGRAPHY 

252. Prairies.— The prairies or treeless plains are 
nearly everywhere covered with a fertile soil, and in a 
moist climate in the wild state they are covered with a 
dense growth of prairie wild grass which formerly sup- 







■» 






^'r^^!^^^'?^'^^^^^-^^i<^!^^9fSSl^(SS^ItliSISS^^^^^^^^M 




E''**^^*'" - 


^^ '^^HMH^h 




fc-.^ 


'^^^^^^^1 


3K' 


5c&fw'' 


tt -^^^K^^^^^^M 




jg'^'^-r-i»,c-«i..' 




m 




BJ^HIHlH^HIHI 



Fiu. 224. View on the Great Western Plains. The plains are mostly treeless 
and have only a light rainfall. The chief industry is grazing herds of 
cattle, sheep, and horses. Tilling the soil is profitable where the surface 
is irrigated. The depression is a buffalo wallow. (U. S. Gteol. Survey.) 



ported great numbers of buffaloes, antelopes, wild horses, 
and other animals. Much of the prairie land of central 
North America is now under cultivation and includes part 
of the famous corn belt of the central region and the wheat 
belt of the north and northwest. 

In the semi-arid region of the middle west, the area just east 
of the Rocky Mountains, the rainfall is not sufficient to grow 
farm products without irrigation, but enough to produce a scanty 
growth of a short but very nutritious grass, which formerly sup- 
ported vast herds of buffalo and which now furnishes food for 



PHYSIOGRAPHIC FEATURES 321 

the great herds of cattle, sheep, and horses grazing on this area 
which extends from the Gulf of Mexico entirely across the United 
States far into Canadian territory. 

Somewhat similar plains occur in the interior of other con- 
tinents. Such are the great wheat lands and grazing plains of 
Argentina in South America, and the great steppes or treeless 
plains of Russia and southern Siberia. 

253. Tundras.— In the far north in both America and the 
Eurasian continent are vast stretches of treeless plains covered 
with a dense growth of mosses and other plants in the short sum- 
mer season. A few feet below this growing vegetation and con- 
tinuing for depths sometimes of a hundred feet or more, the 
ground is perpetually frozen. These frozen plains are known as 
tundras. 

PLATEAUS 

254. Many upland plains are called plateaus. What 
is it that distinguishes a plateau from a plain? It is not 
height alone, for the area of the Great Western Plains east 
of the Rocky Mountains is higher than either the Ozark or 
the Alleghany plateau. 

In passing across the Great Western Plains in any 
direction there is nothing to indicate their elevation, noth- 
ing that appeals to the senses. The higher portions are 
nowhere bordered by land at a perceptibly lower level. 
Even the large rivers flow on the top of the plain. The 
Rocky Mountains and the Black Hills stand high above 
them. Despite the fact that on the plains east of the 
Rocky Mountains the land is several thousand feet above 
sea level, it is much lower than the mountains which border 
them. 

As one crosses the eastern margin of the Alleghany 
plateau, he finds an abrupt descent to the deep valleys 
separating it from the Alleghany ridges. The rivers which 
flow across the plateau flow in deep valleys, and hence by 
comparison the land appears to be high. A plateau is an 

21 



322 



PHYSICAL GEOGRAPHY 



inland plain bordered in part at least by a perceptibly 
lower area, that causes the plateau to appear higher by 
comparison. 




Fia. 225. View in Ausable chasm, New York. A deep chasm cut l>y the 
Ausable river in a hard sandstone rock, (H, W. Brock.) 

255. Canyons. — Plateaus have one characteristic topo- 
graphic feature not common to plains, namely, deep nar- 
row valleys called canyons in the western United States 
and gorges, chasms and glens in the east. The Colorado 
Canyon in Arizona, the Niagara gorge, Ausable Chasm and 
Watkins Glen in New York are alike in being narrow and 
deep. 

The western canyons are generally deeper than the eastern 
gorges because the plateaus are higher, and hence there is a 
greater thickness of rocks through which the stream can cut be- 
fore reaching its grade. Yet there are many small canyons in 
the west on tributaries of the great rivers that are not as deep 
as Niagara Gorge or Watkins Glen. (See figs. 225, 226 and 97.) 

The western canyons are generally narrower in proportion 



PHYSIOGRAPHIC FEATURES 323 

to the depth than the eastern valleys, because (1) the arid 
climate is less favorable to the action of frost and other weather- 
ing agencies which wear away the cliffs and widen the valley in 
the moist climate. (2) The absence of vegetation, due to aridity. 




Fig. 226. Entrance to Grand Canyon of the N, Platte in Wyoming. The 
canyon has a depth of 1000 feet in places in hard sandstones, limestones, 
and granite. 

and the concentration of the rainfall in a few heavy showers, 
favors corrasion in the stream channel, thus deepening the valley 
at the expense of weathering on the sides. (3) The western 
area has been elevated higher and more rapidly. 

Tributary canyons develop in time, and as they increase in 
number, length and size, the plateau is dissected into first table- 
lands, commonly called mesas in the west. The table lands are 
further dissected or worn down to smaller remnants forming 
hills or peaks, called buttes in the west. Name the necessary 
conditions for the formation of canyons. (Figs. 227 and 228.) 

256. Faults.— The rock layers of a plateau may be 
fractured or broken, the plane of fracture being a fissure 
or crack. It frequently happens that the rocks on one side 



324 



PHYSICAL GEOGRAPHY 




Fia. 227. View west of Sheep Mountain, S. Dakota. Mesa in background. 
Cactus flats in the foreground. How does the vegetation indicate dry 
climate? What indication is there of considerable rainfall? (U. G. 
Cornell.) 




Fio. 228. Alkali Buttes, Weston Co., Wyoming. — Mesa in the far background. 
The mesa at one time extended over the entire area. The buttes are 
remnants of the former extension. (U. S. Geol. Survey.) 



PHYSIOGRAPHIC FEATURES 



325 



of a fissure are elevated more than those on the other side. 
Such a displacement of the rocks along a fissure is called 
a fault, and the plane of the fissure becomes a fault plane. 
The direction of this fault plane on the surface is the 
fault line. The projection of the uplifted portion above 
the other side is called the fault scarp. Where the rocks 
are much broken and the fault scarps are numerous, the 
surface of the plateau may be made very irregular. The 
Block Mountains are supposed to be formed in this way 
by the fracturing of the plateau and tilting of the great 
earth blocks along the fault planes. 





1 IT 

Fig. 229. Diagram of fault. AB, fault plane. AC, fault scarp which is 
worn back to DC in 11. 1 is a normal fault. 11 a reverse fault. 



Faults occur in mountains and plains as well as on the 
plateaus, but in many cases they are not noticeable on the surface, 
as after the elevation of the fault scarp the eroding agencies 
wear down the uplifted portion until the two sides are again on 
the same level. A fault line is sometimes indicated on the sur- 
face by a succession of springs which emerge along the line of 
displacement. (Figs. 229 and 230.) 

257. Economic Importance of Plateaus.— The plateaus 
are generally not so productive and populous as plains. 
First, they are likely to suffer more for want of water, as 
the water plane is not so near the surface. Second, the 
soil is frequently not so fertile as in the valley or on the 



326 



PHYSICAL GEOGRAPHY 



low plain. Third, transportation is generally more ex- 
pensive than on the plains, due to distance from the sea 
coast, absence of navigable rivers, and the cost of bridging 
the numerous canyons. 




Fia. 230. Fault at Jamesville, N. Y. Locate the fault plane and determine 
from the way the rocks are bent and broken whether the rocks on the left 
moved up as in 229 11 or down as in 229 1. 



The climate is generally more healthful on the plateau 
than on the low plains, as the air is lighter, purer, and 
freer from malaria. The arid plateaus have and probably 
always will have a scanty population, because of the dif- 
ficulty of getting water. In a moist climate the plateau 



PHYSIOGEAPHIC FEATURES 



327 



may have a prosperous farming population, which, how- 
ever, is generally less dense than on the low plains. 




Fig. 231. Entrance to coal mine in the Alleghany Plateau, Allegany County, 
Md. In a plain country like Illinois coal is lifted to the surface through 
vertical shafts. Here the coal is drawn to the surface on cars on a nearly 
level roadway, extending from the sides of the deep valleys in under the 
plateau. (See also Figs 69 and 185) (Md. Geol. Survey.) 



The Alleghany plateau through Pennsylvania and southward 
is underlain by numerous beds of coal and fire-clay. Many deep 
valleys cut through these beds, and expose them to view on the 
hillsides in favorable position for mining. (See fig. 231, also 
fig. 69.) 

m nearly all plateaus, beds of sandstone, limestone and other 
rocks suitable for building purposes are exposed on the steep 
slopes bordering the valleys, where many quarries have been 
opened for the purpose of obtaining the stone. 



328 PHYSICAL GEOGRAPHY 

DESERTS 

258. The desert is such a striking contrast to the grass- 
and-forest-covered plains and plateaus of the humid areas 
as to seem like a new world. Frequently the first impres- 
sion on visiting a desert is that one is in dreamland where 
things are not what they seem. Lakes appear where there 




Pig. 232. On the Mojave desert near Bagdad, California. There is a great 
scarcity but not absence of life and water. (Detroit Publishing Co.) 

is no water and sometimes seem to run over the moun- 
tains into the sky. Rocks, hills and mountains appear dis- 
torted and unreal. Grave and sombre tints have replaced 
the green leaves and bright flowers. One seems to be look- 
ing through a yellow or brown glass. 

One of the most impressive features of the desert is 
the absence of noise, the apparent absolute stillness. One 
is surprised to hear the ticking of his watch in his pocket, 



PHYSIOGRAPHIC FEATURES 329 

the beating of his heart, and other sounds which in our 
wave-disturbed atmosphere are never heard. (Fig. 232.) 

Sombre, dreary and desolate as the desert may appear 
at first sight, its solitude has charms that often prove ir- 
resistible. No peoples are so wedded to their native soil 
as the nomads of the desert. Rarely indeed do they emi- 
grate either singly or collectively. 

Definition. If it is the absence or scarcity of life due 
to unfavorable conditions that makes a desert, then there 
are three classes of deserts : 

(1) The dry desert, barren for want of sufficient rain; 
(2) the cold desert^ barren because of the low temperature 
and excess of snow which occurs in polar regions and high 
mountains; (3) the wet desert, in mid ocean, where the 
barrenness is caused by the darkness, cold, and pressure. 
The surface of the ocean teems with life, and there is some 
life in the bottom of the deep sea, but the great body of 
the ocean included between these two zones is almost barren 
of life and in that respect it is a great desert. 

The dry deserts, although smaller than the others, are 
the ones commonly meant when the word is used ; that is, 
in addition to the idea of barrenness or scarcity of life, 
the word desert conveys frequently the idea of aridity or 
scarcity of water. Used in this sense, the second and third 
classes of deserts mentioned disappear. 

In the broader sense, a desert is a region conspicuous 
for the scarcity or absence of life, especially vegetable life. 
In a more limited sense, the barrenness is due to scarcity 
of moisture. 

259. Dry Deserts.— The definition sometimes given 
for a desert, that it is a rainless region, is a faulty one as 
no region is entirely rainless and generally there is no 
sharp line of separation between a desert and a semi-arid 
region. Ordinarily less than 20 inches annual rainfall 



330 



PHYSICAL GEOGRAPHY 



means a region too dry to be cultivated and hence classed 
as semi-arid and used for grazing purposes, unless it can be 
irrigated. But other factors than annual rainfall must be 




FiO. 233. Big Bad Lands, S. D. Erosion by heavy rains on soft material. 
The annual rainfall is light but concentrated. The region is desert be- 
cause of the irregular distribution of the rainfall. (U. S. Geol. Survey.) 

considered. Much depends on the distribution of the rain, 
whether it falls in the growing season or not, whether it all 
falls in one or two heavy downpours, or is distributed 
through the year. (Fig. 233.) 

The rains of the desert are generally violent, often accom- 
panied by cloud bursts. The effect produced on the surface is 
to form raging torrents which erode deep gullies or channel-ways 
known as wadies in the Sahara, and as arroyos or barrancas in 
the desert areas of the southwest United States. (See sec. 94.) 
These watercourses, strewn with boulders, sand and driftwood, 
are characteristic features of aridity. (Figs. 234 and 72a.) 

Wind is an important sculpturing and transporting agent in a 
desert region. Hot and dry atmosphere produces a dry sur- 
face to the soil. The absence or scarcity of vegetation causes 
a bare surface susceptible to the action of the winds which blow 



PHYSIOGRAPHIC FEATURES 331 

the dust and sand from place to place. The accumulation of the 
sand forms dunes not unlike those in moist climates. (See sec- 
tion 185.) 

The fine sand and dust carried by the winds against or 
across the surface of the bare rocks acts as a sand blast in 
grinding and wearing the surface, even the hardest of rock sur- 




FlG. 234. Arroyo or stream channel in Arizona. The channel is dry nearly 
all the time, but occasionally it is swept by a torrent. (D. T. McDougal.) 

faces, (Fig. 120a shows some quartz pebbles, among the hardest 
of common rocks, that were worn by the desert sand blast. Com- 
pare them with the stream pebbles and the glacial pebbles.) 

The hard granite rocks of the desert are frequently worn In- 
to weird shapes by the wind-blown sands. (See fig. 191.) 

260. Desert Life.— The life of the desert is in strong 
contrast to that in a humid area. One of the most prom- 
inent characteristics of the life is its scarcity, but not often 



332 



PHYSICAL GEOGRAPHY 



is it wholly absent. Sombre colors prevail in both animal 
and vegetable forms. 

"The life on the desert is peculiarly savage. It is a show of 
teeth in bush and beast and reptile. At every turn one feels the 



gill 


m jLfiaBp|Mjyl 1 li HUy ' 




^" 1P|BS^ 







Fig. 235. Uesert vegetation. Cacti and mesquite near Torres, 
(See also Fig. 227.) (D. T. McDougal.) 



Sonora, Mex. 



presence of the barb and thorn, the jaw and paw, the beak and 
talon, the sting and the poison thereof." (The Desert, Van Dyke, 
Page 27.) 

The more common plants of the American desert are the sage 
brush, greasewood, cactus, yucca, bunch grass and mesquite. 

The animals are the coyote, jack rabbit, antelope, prairie 
dog, rattlesnake, horned toad and Gila monster. (See figs. 235, 
236, 237 and 227.) 

261. Distribution of Deserts.— Probably the largest 
of all the dry deserts is the Sahara of Africa. Extensive 
desert areas occur in central Asia, and in central and west- 
em Australia. There is a narrow strip of desert on the 
west coast of South America, a region that is probably as 
nearly rainless as any on the globe. 



PHYSIOGRAPHIC FEATURES 333 

There are several desert areas in the west and south- 
west United States, areas of considerable extent but grow- 
ing smaller each year. In the older books the Great West- 




I'lG. 236. Sage brush (Artemisia tridentata), the most common plant of tho 
arid plains of Western United States. View near Elko, Nevada. Snow- 
capped Ruby Mountains in the distance. (D. T. McDougal.) 

ern Plains between the Missouri river and the Rocky Moun- 
tains were called the ''Great American Desert." The area 
now supports a large and increasing population. The title 
was next applied to the area west of the mountains, includ- 
ing at first all that vast area between the Rocky Mountains 
and the Sierra Nevadas ; but as it became better known the 
desert portion of this area decreased so rapidly that it does 



334 



PHYSICAL GEOGRAPHY 



not now appear on the United States map at all. There 
are several desert areas of appreciable size in Utah, Nevada, 
New Mexico, Arizona, and southern California, but they 
are separated by productive or partially productive areas 
which are gradually increasing in size at the expense of 
the barren ones. 




Fig. 237. Knight's Temple in Bates Hole, Wyoming. Life in a region of 
slight but concentrated rainfall. Trees along the creek, scanty grass in 
foreground. The hills in the background are nearly devoid of plant life. 
(U. G. Cornell.) 



The desert areas have been greatly diminished by irrigation, 
and other portions have been brought under cultivation by tilling 
the soil. The process known as dry farming is now carried on 
extensively in areas formerly considered barren. In these ways 
a thrifty and industrious population is gradually encroaching up- 
on and thus diminishing the desert areas of the western United 
States. The relatively small portions of the great desert areas 
that have not been thus brought under subjection have been 



PHYSIOGRAPHIC FEATURES 335 

robbed of their former terrors by the horse, the steam and elec- 
tric railways and the automobile. 

MOUNTAINS 

262. Mountains are the most conspicuous features of 
the earth's physiography. We can see part of the ocean 
or part of a large plain, but we cannot see as much of 
either as we can of a mountain. Hence the mountains 
impress us with ideas of vastness, sublimity, and Omni- 
potent power. 

Long ago man, in his imagination, peopled the moun- 
tains with giants, goblins, and dragons so that they were 
objects of dread and were avoided by all. A mountain 
range was then a practically impassable barrier. But in 
the 18th century the ghosts and goblins that had been hov- 
ering in the shadows of superstition began to disappear be- 
fore the bright headlight of scientific discovery and inves- 
tigation and the mountains became objects of attraction 
and study rather than of distrust. People are attracted 
to the mountains in different ways: some for the scenery, 
as nowhere else do we get such grand and inspiring pan- 
oramas; others for the exhilarating atmosphere and the 
fresh, sparkling waters; others for the mineral wealth; 
others for the timber ; others for the fish and game products. 

While mountains are interesting features to all, they are 
especially so to the geologist and the geographer because they 
whisper to him many of the secrets of nature that on the plains 
are concealed beneath the heavy cloak of mantle rock. In the 
mountains this mantle is rent and torn in many places, revealing 
the history of the past in the structure of the underlying rocks. 
The geologist also observes that while the mountains are now 
the highest portions of the earth, they were born in the ocean 
and are truly children of the sea. 

There is no well-defined line of separation between mountains 
and hills or between mountains and plateaus. Mountains are 



336 



PHYSICAL GEOGRAPHY 



higher and larger than hills, but the elevations are comparative. 
Thus the Fourche Mountains at Little Rock, Arkansas, are less 
than 400 feet above sea level, but they are much higher than any 
other area between them and the Gulf. The Blaclc Hills in South 
Dakota are many times higher than the Fourche Mountains, 
much higher even than the Alleghany Mountains, but they are 
dwarfed by the lofty Rocky Mountains to the west and are pop- 
ularly called hills. 




Pia. 238. An anticline, Hancock, Md. formed by the upward bending of 
the strata. (U. S. Geol. Survey.) 

Mountains may be caused by diastrophism, folding, 
faulting, uplifting with erosion, or volcanic action. The 
uplift accompanied by folding is called orogenic to distin- 
guish it from an uplift without folding, epeirogenic. The 
former produces mountain ranges, the latter plateaus which 



PHYSIOGRAPHIC FEATURES 



337 



are dissected by streams into mountains. Volcanic erup- 
tions also build up mountains. (See Volcanoes.) 

263. Folded Mountains.— Most of the great mountains 
are caused by orogenic movements. The foldings are fre- 




FiG. 239. Syncline, three miles west of Hancock, Md. A ti*ough- shaped 
downward bending of the strata. (U. S. Geol. Survey.) 



quently complex and after the surface has been subject to 
erosion for a long time, it is much diversified by the more 
rapid cutting away of the soft layers, which causes the 
hard layers to stand above the surface as mountain ridges 
and hills. 

The crumpling of the layers produces anticline, syn- 
cline and monocline folds. The upward bending forms 
the anticline, the downward bending or trough forms the 
syncline, while a single bending from one level to another 

22 



338 PHYSICAL GEOGRAPHY 

is a monocline. ( Study the diagrams, figs 240 and 242, and 
figs. 238 and 239.) 

After the bending of the rocks, the tops of the anticlines 
form the tops of the mountains or ridges where erosion begins 
most actively, because they are the higher points, and possibly 
also because the rocks there are more shattered and broken. 
Thus valleys form along the anticlines which are cut down more 
rapidly than the first valleys in the synclinal troughs, until in 
time the synclines (the first valleys) are left as mountains high 
above the level of the eroded anticline which now forms the val- 
ley. The accompanying diagrams indicate how erosion has cut 
off the tops of the anticlines until the synclines stand at higher 
levels and form mountains. Many of the present ridges of the 
Alleghany and other mountains are synclines. When first up- 
lifted the mountain ridges were on the anticlines. (Fig, 240.) 




Fia. 240. Diagram illustrating crumpling of the strata in mountain-making. 
A, anticline ; S, syncline ; M, monocline. Dotted lines represent portions 
eroded after the crumpling. 

Parallel ridges and terraced mountains. During the 
erosion of the top of a large anticline which contains alter- 
nating layers of hard and soft rocks, parallel valleys de- 
velop on the softer layers which leaves the harder layers 
standing up as parallel ridges between the valleys. In this 
way there may be formed a succession of ridges, a half 
dozen or more, on a single anticline. Sometimes one hard 
layer is more resistant than another and will therefore 
form a higher ridge, the less resisting layer forming a 
lower parallel ridge which to the observer at a distance 
appears like a terrace on the side of the higher mountain. 
These are called terraced mountains and occur in a num- 
ber of places among the Alleghany ridges. Tussey Moun- 



PHYSIOGRAPHIC FEATURES 



339 



tain along the south side of the great Nittany Valley in 
central Pennsylvania is a terraced mountain. (See fig. 
241.) 




Fig. 241. Tussey Mt., Center Co., Pa. View of a terraced mountain. 
The first ridge or terrace is several hundred feet lower than the one in 
the background and is separated from it by a valley eroded on the softer 
sandstone between the ridges. The stream from the dividing valley flows 
through the water gap in the middle of the picture. 



Canoe Mountains.— Canoe mountains are formed in anticlinal 
and synclinal folds by the ends of the axis dipping below the 
surface, as shown on the accompanying diagrams. They receive 
their name from their resemblance to a large upturned canoe in 
the anticline and an upright canoe in the syncline. After the 
erosion of the top of an anticlinal fold it resembles a canoe with 
the bottom worn off. Deep erosion in the central portion of such 
a fold produces some unique basins or coves cut off or isolated 
from surrounding regions by a mountain-rim. People dwelling 
in such isolated localities are likely to retain habits and customs 
which have disappeared years before in more cosmopolitan 
localities. (Fig. 242.) 

A great mountain range, such as the Alleghany, consists of 
a series of simple and complex folds with often a bewildering 



340 



PHYSICAL GEOGRAPHY 



number of ridges, all of which go to make up the mountain range. 

264. Domed Mountains.— Domed mountains are 
formed by the uplift of a broad, dome-shaped arch instead 
of an elongated one; that is, the layers dip from a point 




Fig. 242. Canoe Mountain which occur.r among the Alleghany 
mountain ridges. H, H, hard rock strata wnlch form ridges 
when intervening softer layers are removed by erosion. The 
upper figure is a syncline in which the hard layers resemble 
a nest of canoes. The lower figure is an anticline in 
which the canoes are inverted and the bottom worn off. 
(After Willis.) 



PHYSIOGRAPHIC FEATURES 341 

or center, instead of from a line as in the anticlinal folds. 
Domed mountains may be simple or complex with the cen- 
tral mass more or less intricately folded or faulted. The 
Adirondack Mountains are an example of complexly 
crumpled domed mountains. They consist of crystalline 
metamorphic and igneous rocks surrounded by sedimen- 
tary layers which dip away from the mountains in all di- 
rections. 

265. Laccolites. — A laccoUte or laccolithic mountain is formed 
by the intrusion of a mass of igneous rock which does not reach 
the surface but spreads out between the layers in a mushroom- 
shaped or umbrella-shaped mass, at the same time pushing or 
bulging up the overlying layers into a dome-shaped mass. 

Like all other mountains, the laccolites generally have a very 
irregular and diversified surface due to the action of eroding 
agents. The Henry Mountains in Utah are good examples of 
laccolites or simple domed mountains. (See fig. 211.) 

266. Block Mountains are caused by fracturing of the earth's 
crust into huge blocks which are then tilted or set on edge, 
thus giving the newly formed mountain a steep slope on one side 
and a quite gentle slope on the other. They are the fragments 
of a broken-up plateau. They occur in the great interior basin 
east of the Sierra Nevada Mountains. 

267. Mountains of Circum-Erosion.— Mountains may be 
formed from a plateau without fracturing by the eroding action 
of streams and their tributaries, which cut the plateau into 
fragments that are called mountains. The larger ones are likely 
to be flat-topped and form table mountains. They are very irre- 
gular in shape and size. Such are the Catskill Mountains, and 
the mountains of western Pennsylvania and West Virginia. (See 
Fig. 299.) 



Volcanic Mountains.— Volcanic mountains con- 
sist of the cinder and lava cones built up around the crater 
of a volcano. Some of the loftiest mountain peaks in the 
world are formed in this way. While the volcano is still 
active the cone may be quite symmetrical, but as soon as 
it becomes extinct the effects of the eroding agencies are 



342 PHYSICAL GEOGRAPHY 

shown in the gullies and valleys that form on the slope. 
When the other portion of the mountain is worn away the 
central neck or core of the volcano may still form a prom- 
inent mountain mass. (See figs. 207, 208, and 209.) Mt. 
Shasta, Mt. Hood, Mt. Baker and many other high moun- 
tain peaks of the western United States are volcanic 
mountains. 

269. Life History of Mountains. — All the great mountain 
ranges are born in the sea, having a beginning in the accumula- 
tion of a great mass of sediments on the ocean bottom bordering 
the continents. Since the margin of the ocean is shallow, in 
order to have a great thickness of deposits, it follows that the 
bottom is sinking while the sediments accumulate. After a long 
period of subsidence the uplift or movement in the opposite di- 
rection begins, when the sea bottom sediments are folded and 
elevated as mountains high above the sea. Then the agencies 
of erosion, the sculpturing agencies, begin their work. The accu- 
mulation of the thick bed of sediments might be called the 
embryonic stage of development. 

The youthful stage is that immediately following the eleva- 
tion above the sea in which the mountain has the regular 
slopes of the first uplift. The rain and the other weathering 
agencies soon start gulches and valleys which change the smooth 
slopes into very rugged ones. The more rapid erosion of the 
softer strata causes the harder layers to stand out as hills, ridges 
and irregularities on the surface. During this stage waterfalls, 
rapids, canyons, gorges, steep cliffs, and talus slopes are formed. 
In the higher mountains, snow fields with accompanying glaciers 
and glacial erosion modify the surface, while avalanches, land- 
slides and earthquakes are accompanying features. (See figs. 
104, 114, and 115.) 

Mature stage. The mountains pass from youth to maturity 
as the softer parts are worn away, as the streams are established, 
as the ridges and peaks reach great elevation and ruggedness, 
and as the divides become narrow and well defined. The valleys 
are still deep but wider than in the youthful stage, while fiood 
plains are forming on the short grade-level stretches, talus slopes 
are becoming larger and extending nearer the tops of the cliffs, 
waterfalls are fewer, and caves are forming in the limestone 



PHYSIOGRAPHIC FEATURES 343 

strata. The larger streams form water gaps where they cut 
through the ridges and the shifting of the smaller streams leaves 
many wind gaps which serve as passes for highways. 

Old age begins when the talus slopes extend to the tops of 
the ridges and peaks, which are crumbling and being washed 
down into the valleys, when the flood plains are expanding and 
the streams meandering in their courses. The heights of the 
mountains are lowered by erosion from the summit and the hill- 
sides are made less steep by erosion at the top and filling in at 
the bottom. The talus slopes increase in size until they reach 
and cover the tops o*f the cliffs. The rocky, barren cliffs of 
maturity give way to soil-covered, farm-covered slopes in old age. 

The region about New York City, Philadelphia and Balti- 
more represents extreme old age of mountains, where they have 
all been cut down to low hills or even plains in places. The final 
stage of mountain erosion, like that of plains and plateaus, is the 
peneplain (Sec. 216) which may in time be brought to base level. 
Over the peneplain the harder and more resistant rocks stand 
forth as relict mountains or monadnoclis, the remnants of the 
former high peaks and ridges. (See fig. 194.) 

Height. The height of a mountain or mountain range 
at any time depends upon a number of factors, such as the 
rate of elevation, the character of the rocks in the moun- 
tain mass, the age of the mountains, and the climate. 

In many places in the Alleghany Mountains, a thick- 
ness of several miles of rock has been eroded, but it does 
not follow that the mountains were several miles higher 
than at present, because erosion was taking place while 
elevation was going on. Whether the mountains were ever 
higher than at present and how much higher depends on 
the relative rate of activity of the eroding and elevating 
forces. 

In general the young and mature mountains are the 
higher mountains, higher because they are young and ma- 
ture. The age of mountains is commonly reckoned from 
the time of their first uplift from the sea bottom, which is 
indicated by the age of the upper strata or the newest sedi- 



344 PHYSICAL GEOGRAPHY 

ments that take part in the folding of the mountains. It 
should be considered that some of the mountain ranges 
have been worn down and re-elevated one or more times 
since the first uplift. There is good reason for thinking 
that the Alleghany Mountains have been re-elevated at 
least twice. 

270. Mountains a? Barriers.— Mountains act as barriers in 
the distribution of moisture. In crossing high mountains, 
moisture-laden winds lose most of the water on the windward 
side and pass down the lee side as drying winds. Thus the west 
slope of the Andes in the trade wind belt receives almost no 
moisture, as it is nearly all precipitated on the east side of the 
mountains and helps to form that greatest river in the world, the 
Amazon. 

The higher the mountain, the more effectual barrier is it to 
the vegetation and the lower forms of animal life. Very few of 
the plants that grow on the plains or in the valleys could by 
natural means cross such mountains as the Rocky Mountains or 
the Andes. Hence the native plants — the wild flowers, shrubs 
and trees — are quite different on the two sides of these moun- 
tains. The same is true of many animal forms. Some of the 
hardier and more roving forms of life find the mountains an 
obstruction, but like man they can and do cross them. 

The difficulties attending the crossing of mountains are 
frequently a decided check to the free intercourse of the peo- 
ple on both sides and the partial isolation in sequestered moun- 
tain valleys is liable to cause a very provincial community, in 
which habits and customs of past decades are preserved. In 
many of the deep valleys or coves in the Alleghany Mountains 
one may see the customs of fifty years or more ago with little 
change or modification. In some places the old style wagons with 
wooden axles, linch pins and tar buckets are still in use. These 
wagons were in common use fifty years ago, but probably none 
of the readers of this book ever saw one unless he has been visit- 
ing in some of the sequestered mountain valleys. 

271. Mountain Climate.— The climate of the moun- 
tains is different from that of the surrounding plains and 
valleys. So marked are the differences on mountains of 



PHYSIOGRAPHIC FEATURES 345 

even moderate height from surrounding lowland areas 
as to cause a difference of plant and animal life. The cli- 
mate is likely to prove more moist than on the lowlands, 
but in the region of prevailing winds this may prove true 
of only the windward side of the mountains while the lee 
side may be dry and barren. The east side of the Andes 
in the trade wind belt has a very heavy precipitation 
while the west side is rainless. In the Himalayas the north 
slopes are dry and are bordered by an arid region, while 
the south slopes have an exceptionally heavy precipitation, 
the heaviest in the world. 

The heavy rainfall and snow fall of the mountains are 
being used in many localities in the western United States 
to furnish w^ater for irrigating the surrounding semi-arid 
plains and plateaus. The mountains and plateaus are im- 
portant factors in inducing rainfall over the continents. 
The winds passing over the ocean and plains are warmed 
and absorb moisture; in climbing the mountains they are 
cooled and precipitate the moisture. 

The change in temperature found in ascending moun- 
tains is quite marked. Not only is the average tempera- 
ture lower, but the daily range of temperature is greater. 
Night on the mountains is alw^ays cool and on the highest 
mountains is alw^ays cold even in the summer season. 

272. Economic Features of Mountains.— The moun- 
tains are the great health resorts, furnishing sites for sum- 
mer homes, and hotels, where fresh air, pure water, and 
wholesome exercise are obtained by multitudes from the 
crowded cities and towns. 

Timber supply. Some of the mountains are utilized 
as forest preserves and more of them should come under 
government control for this purpose. The most rugged 
mountains can never be cultivated to advantage and where 
forests are not preserved or cared for, the mountains be- 



346 PHYSICAL GEOGRAPHY 

come fire-swept, barren wastes instead of profitable and 
attractive woodlands. 

The forests may not only be a profitable and continued 
source of lumber supply, but at the same time furnish 
game preserves where wild game and fish may flourish. 
They may at the same time furnish the much needed regu- 
lator of water supply to the streams, prevent disastrous 
floods in one 5*eason and dry stream beds in another, by 
conserving the heavy rainfall and distributing it through 
the dry season. 

Mineral wealth. Mountains are the sources of much 
of the mineral wealth, due in part to the fact that deep 
erosion has exposed the deep seated rocks, thus causing 
the exposure of a great thickness and range of rocks to 
the view of the miner. It is also in large measure due to 
the fracturing and metamorphism of the rocks during the 
mountain-making process, thus producing more veins and 
greater concentration of valuable minerals in veins. Most 
of the mines of gold, silver, quicksilver, and other metals 
are located in the mountains, occurring in old mountains 
as well as in young or mature ones. 

Building stone. There are large exposures of rocks of 
many kinds in the mountain areas because of the folding 
and erosion, and hence they furnish sites for stone quar- 
ries. However, the largest and most productive rock quar- 
ries are not in the mountains. The limestone quarries of 
Indiana and Illinois, the sandstone quarries of Ohio and 
Connecticut, the granite quarries of eastern Massachusetts 
and Southern Maine are all remote from mountains. 
What reasons can you give for such a condition ? 

EEFERENCES 

Plains : 

Johnson, High Plains of the United States, 21st An. Rept. 
U. S. Geol. Survey, Pt. VII, p. 601. 



PHYSIOGRAPHIC FEATURES 347 

Johnson, High Plains of the United States, Natl. Geog. Mag., 

Vol. IX, p. 493. 
Salisbury, The Physical Geography of New Jersey, N. J. 

Geol. Survey, Trenton, 1895. 
Darton, The Great Plains of the Central United States, An. 

Rept. U. S. Geol. Surv., also Scottish Geog. Mag., Jan., 

1906. 
Deserts : 

Van Dyke, The Desert, Chas. Scribner's Sons, 1901. 
MacDougal, Desert Vegetation, Publication of Carnegie Insti- 
tute. 
National Geographic Magazine, April, 1904, The American 

Deserts. 
Davis, A Temporary Sahara, Jour. Sch. Geog., Vol. IV, 1900. 
Piatt, The Sahara, Jour. Sch. Geog., Vol. IV, 1900. 
Herbertson, Man and His Work. 
Plateaus : 

Powell, Canyons of the Colorado, Flood & Vincent, Meadville, 

Pa. 
Dutton, The Colorado Canyon, Mon. II, U. S. Geol. Survey. 
Campbell and Mendenhall, Plateau of West Virginia, 17th 

An. Rept. U. S. Geol. Surv. 
Hodge, The Enchanted Mesa, Natl. Geog. Mag., Vol. VIII, 

1897, p. 273. 
Mountains : 

LeConte, Theories of the Origin of Mountain Ranges, Jour. 

Geol., Vol. I, p. 542. 
Willis, Mechanics of Appalachian Structure, 13th An. Rept., 

iPt. II, U. S. Geol. Survey, p. 217. 
Willis, Northern Appalachians, Natl. Geog. Mon., Am. Book 

Co., 1895. 
Hayes, Southern Appalachians, Natl. Geog. Mon., Am. Book 

Co., 1895. 
Davis, Southern New England, Natl. Geog. Mon., Am. Book 

Co., 1895. 
Davis, Rivers and Valleys of Pennsylvania, Nat. Geog. Mag., 

Vol. I, p. 183. 
Geikie, Classification of Mountains, Scot. Geog. Mag., v. 17, 

Sept., 1901. 
Lubbock, Scenery of Switzerland, MacMillan Company, 1896. 
Cross, Laccolitic Mountain Groups, 14th An. Rept. U. S. Geol. 

Surv., Pt. II, p. 165. 



CHAPTER X 
THE ATMOSPHERE 

273. The atmosphere is the gaseous portion of the 
earth which surrounds the liquid and solid portions. 
Floating in suspension in the atmosphere are large but 
variable quantities of moisture in the form of invisible 
vapor and also condensed vapor in the form of clouds, fog 
or mist, along with many dust particles and microscopic 
organisms. All this material while in the atmosphere may 
be considered as a part of it, in the same way that the por- 
tions of the gases which penetrate the water and the land 
may be considered as portions of the water and land- 
spheres for the time. The air is as truly a part of the 
earth as the water or the land. It even approaches, pos- 
sibly exceeds them in volume, although it is less in mass. 

The science which treats of the atmosphere, its posi- 
tion, functions, phenomena and laws governing them is 
called meteorology. 

274. Origin of the Atmosphere.— According to the nebular 
hypothesis, all portions of the earth were at one time in a gas- 
eous condition, but as the gases cooled and contracted, the 
greater part became liquid, most of which on further cooling be- 
came solid. The air is the portion which still remains gaseous. 
The physical state of matter is partly a question of temperature. 
At 32 degrees F. (F.=Fahrenheit, the thermometer in com- 
mon use) water freezes and becomes solid; at 212 degrees it 
boils and becomes a gas. Some substances, as metallic mercury, 
freeze at a much lower temperature,— 40 degrees F., while many 
of the common rocks freeze solid at temperatures as high as 
3,000 degrees to 5,000 degrees F. Portions of the present at- 
mosphere have been reduced to a liquid by lowering the tempera- 

348 



THE ATMOSPHERE 349 

ture and increasing the pressure, but at ordinary temperatures 
it remains gaseous. 

275. Function of the Air.— In the economy of the 
earth the atmosphere serves many important functions, 
such as (1) diffusing light; (2) conducting sound; (3) 
retaining heat; (4) supporting life in many ways; (5) 
supporting combustion; (6) moving ships; (7) driving 
windmills; (8) reducing the weight of bodies submerged 
in it, thus making it possible for some animals to walk and 
others to fly; (9) producing waves and ocean currents; 
(10) moving sand and dust and wearing away rock; (11) 
distributing moisture and heat. What other functions 
can you name? Can you see the air? Can you feel it? 
Can you weigh it? 

276. Composition of the Atmosphere.— The atmos- 
phere consists of a mixture of nitrogen and oxygen in the 
proportion of nearly four parts of nitrogen to one of oxy- 
gen along with small but variable quantities of carbonic 
acid gas, water vapor, argon, crypton, helium, and prob- 
ably other rare but yet unknown gases, besides variable 
quantities of dust particles. 

Nitrogen, which forms about four fifths of the bulk of the 
atmosphere, is one of the most inert of the gases and so far 
as life is concerned its chief function appears to be to dilute 
the oxygen. It has little or no tendency to combine with the 
other elements under normal conditions, but certain plants, such 
as clover, have the power of secreting it in their roots in the 
form of nitrates which greatly enrich the soil. Nitrogen com- 
bined chemically with oxygen and water forms nitric acid. If 
this comes in contact with soda or potash it combines with them 
forming compounds called nitrates. 

Oxygen forms about one fifth of the atmosphere and is much 
more active and aggressive in its character than the nitrogen. 
It is the chief agent in combustion; in fact nearly all burning 
consists of the chemical union of oxygen with carbon, forming 
carbon dioxide and other gases, whether it be in fires, where it 



350 PHYSICAL GEOGRAPHY 

produces heat and light, or in our own bodies where it forms 
heat, but not light. Oxygen is ever active also in combining 
with metals and minerals in the rocks of the earth. A knife left 
on the ground in a few days is covered with rust; in a few 
months it crumbles to fragments, eaten up by the rust, in other 
words, by the oxygen of the atmosphere. Oxygen forms half of 
the rocks of the earth's crust, and eight-ninths of the water. It 
is being taken from the atmosphere by animals, by fires, by the 
rusting of rocks and minerals. It is being returned by plants 
which take the carbon dioxide gas from the air and separate it 
into the elements, carbon, which makes new compounds forming 
part of the plant, and oxygen, which is returned to the air. It 
is also set free by certain chemical changes in the rocks. 

Carbon dioxide or carbonic acid gas (CO2) forms a small but 
important part of the air. The proportion varies greatly at 
different places and times but averages about 0.03%. 

It is one of the chief heating agents in the atmosphere and 
thus has considerable influence on the climate, owing to a varia- 
tion in the proportion present from time to time. It is a 
denser, heavier gas than nitrogen or oxygen and absorbs and 
holds the heat from the sun's rays, thus serving as a blanket 
to warm the earth. It is directly necessary to plant life, furnish- 
ing the most important article of food for the plant and indirectly 
necessary to animal life. Why? When dissolved in water it 
becomes an active agent of solution and disintegration in the 
rocks, especially the limestones. 

Carbonic acid is added to the atmosphere by the breath of 
animals, by the decay and combustion of all animal and vegetable 
matter, by volcanoes, by carbonated springs and from the sea 
water. It is extracted from the air by plants which fix the car- 
bon in their tissue and giv« the oxygen back to the air, by the 
disintegration of the rocks in which it combines with other ma- 
terials to form carbonates, and it is absorbed by the sea water 
and fresh waters where these are not already saturated with it. 
Owing to this circulation through the rocks and the sea, the 
proportion of this gas in the air varies sufficiently from one geo- 
logic age to another, it is thought, to affect the climate materially. 

Water vapor is a small, variable, but very important consti- 
tuent of the air. The water in an invisible gaseous condition is 
absorbed by the atmosphere from the surface of the ocean, other 



THE ATMOSPHERE 351 

bodies of water, and the moist land. The gases ejected from 
volcanoes also supply large quantities of vapor to the air. An- 
other source of supply is in combustion and as exhalation from 
the lungs of animals. 

The water in the invisible gaseous state is carried by the 
winds over the continents, where, condensing in clouds, it is pre- 
cipitated in the form of rain or snow to fall on the earth and 
flow back to the ocean again to start on a similar circuit. The 
circulation of water in this way is essential to all life — in the 
ocean, in the air, and on the land. A large per cent of all living 
plant and animal matter consists of water and there must be a 
constant renewal of the water supply in order to retain the life. 

Dust is another important constituent of the air which is 
everywhere present but most abundant over the land areas near 
the surface in dry weather. Why? It consists of exceedingly 
fine particles of pulverized rock carried up by the winds or 
blown out from volcanoes, of particles of unconsumed fuel in the 
smoke, of living germs in the form of bacteria and microbes, or 
of decayed plant and animal tissue. In a ray of sunshine passing 
through a small opening into a dark room, one can see a vast 
number of the dust particles, but there is a much larger number 
invisible because so very small. 

The dust of the air is thought to be an important aid in the 
precipitation of moisture. The dust particle becomes a center 
of condensation of moisture until there is sufficient to form the 
raindrop or the snowflake, which then falls to the earth. It acts 
with the carbonic acid and the moisture in heating the air, as 
each little dust particle becomes a little furnace or reservoir of 
heat which it radiates in all directions. The bacterial portior 
aids decomposition and the spread of disease. It influences the 
color of the sky, the brilliant red sunset being due largely to the 
dust particles in the air. 

Dust is sometimes classed as an impurity in the atmosphere, 
but since it is always present, and serves a useful if not a neces- 
sary purpose for the support of life, it is properly one of the 
constituents of the air. 

277. Pressure of the Air.- Because the pressure of 
the air is exerted equally in all directions, it long remained 
unperceived. If we should exhaust all the air from under- 
neath a scale pan, we would find a weight of air on the 



352 



PHYSICAL GEOGRAPHY 



pan of about fifteen pounds to the square inch or a little 
more than a ton to the square foot, that is, provided the 
weight were taken at sea level ; with change of temperature 
or change of elevation the pressure would change. It 
would increase if taken below sea level and decrease if 
taken above. It would decrease with an increase in the 
temperature and increase with a decrease in temperature 
because the warm air expands and is therefore 
lighter than the same volume of cold air subject 
to the same pressure and the cold air contracts 
and is hence heavier than a similar volume of 
warm air. 

Weight on the earth is the effect of gravity 
which tends to draw all bodies toward the center of 
the earth, hence the weight of the atmosphere at 
any point is its vertical pressure, but owing to the 
extreme mobility, this pressure due to weight is 
exerted equally in all directions, so that under ordi- 
nary conditions we do not find the scale pan pressed 
downward by air, because there is the same pressure 
underneath as on top. The pressure of ten tons or 
more on the outside of the human body is not felt 
because there is a corresponding pressure on the in- 
side. The weight of a cubic foot of air at sea level 
at a temperature of 60 degrees is .075 lbs., while the 
pressure exerted by this cubic foot would be more 
than a ton on each side. 

278. Barometer.— The instrument com- 
monly used in weighing the air, that is, de- 
termining the pre.ssure, is a barometer, which in 
its j-implest form consists of a glass tube closed 
at one end, filled with mercury and inverted in 
^, a cup of the same metal. If this is done at 

t Hi. '24'.i. • 

Mercury sea Icvcl the mcrcury will settle in the tube until 
barometer, ^hc top of it is .ibout 30 iuchcs abovc the level 

common 

form. of that in the cup. Now a column of mercury 



THE ATMOSPHERE 353 

30 inches high weighs 15 pounds per square inch at the 
base of the cohimn, hence the weight of the air pressing 
on the surface of the cup outside of the tube must be equal 
to 15 pounds per square inch, since the two balance each 
other. Why is mercury used? Why not some other 
liquid? If water were used how long a tube would it 
require ? 

An aneroid barometer differs from a mercurial one in substi- 
tuting for the column of mercury a small corrugated metallic 
box from which the air has been exhausted as 
nearly as possible. The surface of this box is 
connected through a series of levers to a needle 
in such a way that when the sides of the box 
are pressed in, as would be the case with an 
increase of pressure in the air, the needle will 
turn around on a dial like the hand of a watch, 
and when the pressure decreases the needle 
moves in the opposite direction. The dial of 
the aneroid is marked so that movements of j, .^^^ ^^^ 
the needle will indicate the number of feet the roid barometer 

barometer has been taken up or down, or it for measuring 

may be marked in inches to correspond to the elevations. See 

movements of the mercury in the other baro- ^^^' ^"^^ ^"^ ^"' 

„ ^ .^ „ , , ^, , tenor construc- 

meter; m fact it generally has both scales. ^j^^ 

Because it can be carried in the pocket like a 

watch, and is much more convenient for ordinary use than the 

bulky mercurial barometer. 

279. Density of the Air.— If a barometer were car- 
ried up* a mountain the mercury would gradually fall in 
the tube, because of decrease in the pressure of the atmos- 
phere, until at an elevation of 3.4 miles above the sea there 
would be only 15 inches in the tube ; that is, at that height 
the air would be only half as dense or heavy as at sea 
level. At greater elevations the density has been esti- 
mated as follows: 



23 




354 



PHYSICAL GEOGRAPHY 



Elevation 6.8 miles, density ^ and barometer 7.5 inches 

10.2 " " % " 3.75 

13.6 " " T^ " 1.87 

17.0 " " ^V " -95 

This estimate is based on the assumption that the den- 
sity decreases in the upper atmosphere at the same rate 
that it does in the lower atmosphere ; that is, a decrease of 
one-half for every 3.4 miles ascent. 

280. Height of the Atmosphere.- The upper limit of 
the atmosphere is not known, but various estimates have 
placed it at heights varying from 50 to "500 miles. It prob- 
ably extends much higher than either figure ; in fact, there 
is good reason for thinking that it extends as far as the 
outer limit of the zone of control of the earth's gravity. 
But if the decrease in density approximates that indicated 
in the above table it becomes so rare as to be difficult of 
detection by any available means far short of 200 miles. 

281. Pressure Curve.— If the barometer readings are 
recorded at any point for several days it will be seen that 
there is considerable variation. If the barometer should 
be read for several different hours each day for several 




Fig. 245. A barogram or pressure curve made by a barograph during the 
passage of a cyclone followed by a cold wave. Notice the rapid fall and 
rise of pressure. 



days and the results plotted on cross-section paper, the re- 
sult would be the pressure curve. This pressure curve is 
plotted even more accurately by an instrument called the 



THE ATMOSPHERE 



355 



barograph. (See fig. 246.) The curve made by the baro- 
graph is called a barogram. (Fig. 245.) 

282. Barograph.— The barograph used by the U. S, Weather 
Bureau is based on the principle of the aneroid instead of the 
mercurial barometer. It consists of a corrugated iron box (B) 
from which the air has been exhausted so that an increase in the 




Fig. 246. A barograph. The pen traces a line on the paper which is moved 
by clockwork. If the pressure were uniform the line traced would be 
parallel to the horizontal lines on the paper. 



pressure of the air depresses the surface of the box and a de- 
crease in the pressure causes a corresponding elevation in the 
surface produced by a spring inside the box. These movements 
of the surface are magnified by a series of compound levers to 
which is attached a pen so adjusted as to leave its trace on 
the roll of cross-ruled paper which is moved by clockwork. 
This shows conclusively that the pressure at any place at any 
one time is dependent on the condition of the weather. If the 
effect of the weather on the barometer is known, then the process 
may be reversed and the record of the barometer may be taken 
as the indication of the weather conditions; and if ajong with 



356 PHYSICAL GEOGRAPHY 

this, the regular movements of the atmosphere are considered, 
the weather conditions may often be foretold for some time 
ahead. 

283. Isobars.— In order to compare barometer read- 
ings from different localities to determine probable weather 
conditions, it is necessary to make corrections for the dif- 
ferences in elevation of the places and another for the dif- 
ferences in temperature. Tables have been made out for 
this purpose so that having the reading of the barometer 
and the thennometer, and knowing the elevation of the 
point above sea level, it is only necessary to turn to the 
tables and make the necessary additions or subtractions to 
reduce the readings from different places to a common 
plane, which by common consent is taken as sea level. The 
records that are given on the government weather maps 
are all the corrected sea-level readings at the temperature 
of 32 degrees F. 

In order to bring out graphically the results obtained 
from barometric readings at many different stations, lines 
representing variations in pressure of one-tenth of an inch 
are drawn through points having the same barometric 
pressure. Such lines are called isobars (meaning equal 
pressure), and are shown on the daily weather maps by 
continuous black lines. Compare carefully several daily 
isobaric charts or weather maps with each other and with 
the isobaric chart of the world for the year. (See fig. 254.) 

284. Barometric Gradients.— On the daily weather map most 
of the isobars are more or less concentric around centers, some 
of which are marked high and some marked low, meaning high 
pressure and low pressure. The barometric gradient is the rate 
at which the air pressure changes from place to place and 
particularly between the high and the low. It indicates the 
direction in which the atmosphere tends to move, namely, 
from the high to the low, and the steeper the grade, that is, 
the greater the number of the isobars, the more rapidly the 



THE ATMOSPHERE 357 

air moves and hence the stronger is the wind. Test this by- 
comparing current weather maps on a windy day and a calm one. 

285. Temperature and Heat.— Temperature is the 
measure of the heat energy of any body. The instrument 
used for recording the temperature is called a thermometer 
(heat measure). In the common house-thermometer mer- 
cury rises and falls through a capillary tube from expan- 
sion and contraction with increasing and decreasing tern-* 
perature, and generally indicates increase and decrease of 
heat. However, many bodies are capable of absorbing or 
giving off considerable heat without any perceptible change 
of temperature. Thus when heat is applied to ice, the 
temperature does not vary until all the ice has been changed 
to water. The heat required to change a pound of ice to 
water at the same temperature would raise the pound of 
water from 32 degrees to 174 degrees F. This is called 
latent heat of fusion, and will be given off again before 
the water will freeze. It requires a large increment of 
heat to change water from a liquid at 212 degrees to a 
vapor at the same temperature, the latent heat of vapori- 
zation. (This is technically expressed in heat units called 
calories. A calorie is the amount of heat required to raise 
a gram of water one degree Centigrade. The latent heat 
of vaporization of water is 536.6 calories.) Why does 
sprinkling the lawn or the street on a hot summer day cool 
the air? Why is it generally warmer in winter and cooler 
in summer on the sea shore than in the interior? Why is 
it sometimes warmer after a rain than before? 

286. Thermometer.- The temperature of the air is 
usually measured by a mercury thermometer, which con- 
sists of a small bulb filled with metallic mercury that is 
attached to a capillary tube in which the mercury rises 
when it is heated and in which it sinks when it is cooled. 
On the tube or on a fiat surface to which the tube is at- 



358 



PHYSICAL GEOGRAPHY 



taehed is a scale marked off in degrees from which is read 
the number corresponding to the height of the mercury in 
the tube. The mercury thermometer may be used for 
measuring temperatures between — 40 degrees, the freezing 
point of mercury, and 648 degrees F., the boiling point. 
For lower temperatures some other liquid as alcohol or 
ether is used ; for higher temperatures some more resistant 
metal or metals or some other device is used. Describe a 
maximum and a minimum thermometer and the Fahren- 
heit, Centigrade, and absolute scales for grading ther- 
mometers, if these instruments are at hand. 

287. Temperature Curve. —The variation in temper- 
ature at any place from time to time may be represented 
by a temperature curve as shown in fig. 247. The curve 




Fig. 247. Thermogram, a temperature curved formed by the thermograph. 



may be constructed for any period of time for which the 
thermometer readings have been taken. The thermograph 
automatically records such a curve with greater accuracy 
of detail than can be shown on one drawn from thermo- 
meter readings. Study curves of this kind if available, 
and note the time of day when the maximum and mini- 
mum temperatures occur. Each student should plot a 
temperature curve for a week or a month from data ob- 
tained by reading the thermometer and recording the tem- 
perature 3 or more times each day. 



THE ATMOSPHERE 



359 



A thermograph is an instrument for recording automatically 
the temperature for a fixed period of time in much the same way 
as a barograph records pressure. The record of the thermograph 
is a thermogram or temperature curve. (See fig. 247.) 




Fig. 248. Thermograph, an instrument for recording temperature changes. 



288. Sources of Heat.— The three sources of heat sup- 
ply on the earth are (1) the sun, (2) the stars and other 
heavenly bodies, (3) the internal heat of the earth. In 
quantity the last two are insignificant when compared with 
that from the sun, which is the source of not only nearly 
all the heat, but the light, and other forms of energy as 
well. The radiant energy that comes from the sun is 
known as insolation, which manifests itself on the earth in 
part as heat, in part as light, and probably in several other 
forms of energy. The earth receives about one two-bil- 
lionth portion of the sun's insolation and a large part of 
that received is not retained but is reflected or radiated 
off into space. 



360 PHYSICAL GEOGRAPHY 

A small portion of the sun's rays, as they pass through the 
atmosphere, are intercepted by the dust particles and the heavier 
gases and changed to sensible heat, but the greater part passes 
directly through to the. surface of the earth, where a portion is 
absorbed by the rock-and-water-surface and changed to latent or 
radiant heat and a portion is reflected back through the atmos- 
phere. The proportion of rays absorbed to those reflected, varies 
greatly with different surfaces. More are reflected from a water 
surface than from rock and more from light-colored rocks than 
from dark-colored ones. There is likewise a wide variation de- 
pending on the angle of inclination of the rays to the surface. 
The greatest percentage of insolation is absorbed under the ver- 
tical rays and as they vary from the vertical there is an increas- 
ing proportion reflected until the tangent rays, such as those at 
sunrise and sunset, are nearly all either reflected or else pass 
directly through the atmosphere into space beyond. 

289. Temperature of the Air.— The atmosphere is 
warmed (1) by the direct insolation of the sun, (2) from 
the heated surface of the earth by radiation, by conduc- 
tion, and by convection, and (3) by compression as shown 
when pumping air into a bicycle tire. When the air 
descends from altitudes of less pressure to regions of greater 
pressure it is compressed and warmed, as in the high pres- 
sure areas and where the air flows from the mountain top 
into the valley or plain ; (4) by precipitation. The great 
quantity of heat required to change water to vapor remains 
latent in the vapor and is given oft* when the moisture is 
precipitated. 

The air is cooled (1) by radiation of its heat into space; 
(2) by conduction when it comes into contact with the 
cooler earth or body of water; (3) by expansion as when 
it flows out of a bicycle tire or when it rises from a region 
of greater to one of less pressure as in ascending the moun- 
tain or in the rising currents in the low pressure areas; 
(4) by convection, as by the descent of the cold currents 
to replace the rising currents of heated air; (5) by evapor- 



THE ATMOSPHERE 361 

ation. How? The heating of the air by compression and 
the cooling by expansion is known as adiahatic heating and 
cooling. 

290. Elements Affecting the Temperature of the Air. 

— The distribution of solar heat is determined by the suc- 
cession of day and night and of the seasons. The atmos- 
phere and the earth underneath it receive heat directly 
from the sun during the day, receiving the greatest quan- 
tity at midday when the rays are most nearly vertical. 
The temperature, however, is the highest from one to two 
hours after noon. Why? There is more or less regular 
decrease in the heat received from the sun, as one goes 
from the tropics toward the poles. Why? 

Efect of pressure. In areas of high pressure, the air is being 
warmed because it is under pressure and in areas of low pres- 
sure, it is being cooled because it is rising and expanding. How- 
ever, the temperature of the air near the earth is higher in the 
low pressure areas than in the high; in fact, that is the reason 
why it is low, because the temperature is higher and hence the 
air is lighter and is crowded up by the inward pressure of the 
surrounding heavier air; the temperature is kept high because 
the air moves into the center along the surface of the earth, 
where it is warmed by conduction and frequently by precipita- 
tion of the moisture; it cools by expansion as it rises, but it is 
then beyond the reach of our senses. The air is cooler and feels 
cooler in the high centers because it is descending from the 
higher altitudes, where it is much colder. The clear air of the 
high center favors radiation of heat and in that way lowers the 
temperature. Cold waves come with the high centers in the 
winter. The fact that the air is warmed in the high and cooled 
in the low is shown in two ways, by comparing the temperature 
taken at high altitudes, by means of a kite or balloon, and second 
by noting that in the highs the air is clear and generally free 
from clouds and rain, while in the lows, it is cloudy and fre- 
quently precipitates rain or snow. Cooling the air condenses the 
moisture and warming it increases its capacity for moisture, 
causing it to dissolve the clouds instead of condensing them. 
(See Sec. 296 and 314 for explanation of high and low pressures.) 



362 PHYSICAL GEOGRAPHY 

Efect of latitude. The seasonal changes are most marked at 
and near the poles and least at and near the equator, due to the 
fact of less variation in the inclination of the sun's rays in the 
tropics and greater uniformity in the distribution of sunlight 
and darkness. 

The temperature decreases with an increase of latitude at 
the average rate of about 1 degree F. for 1 degree or about 70 
miles of latitude. 

Efect of altitude. The temperature decreases with altitude 
at the average rate of about 1 degree F. for every 300 feet of 
ascent; hence, at the equator one would find about the same 
change in temperature by ascending a mountain six miles high 
as in going north or south 1000 times this distance. The reason 
for the rapid decrease in temperature in ascending the moun- 
tain is that the air is less dense, and contains less carbon dioxide, 
water vapor, and dust particles that serve to warm the air at 
lower altitudes. 

Effect of 'Wa.ter.— Bodies of Water tend to equalize 
the temperature of the region bordering them, because they 
absorb heat less rapidly than the land, do not become so 
hot during the day or during the summer season, and 
since they part with their heat less rapidly than the land, 
they do not cool as quickly; hence the water is cooler than 
the land in the summer and warmer in the winter and ex- 
ercises a corresponding effect on the atmosphere that comes 
in contact with it Therefore points along the sea coast 
have a more uniform climate than inland points. The 
same thing is true in a less degree of the lakes, especially 
the larger lakes. Even the Finger lakes of central New 
York temper the climate on their shores, but their influence 
does not extend so far away as is the case with larger 
bodies of water. 

The chief reasons for the tempering influence of the 
water on the air are (1) the greater specific heat of water 
so that it requires about twice as much heat to raise a 
given volume of water one degree as it does a similar vol- 



THE ATMOSPHERE 363 

ume of earth; (2) being more mobile the water warmed 
in one place is carried by currents to another and colder 
part of the ocean, and since cold water is denser and 
heavier, the water cooled at the surface sinks to the bot- 
tom and the surface is not frozen until the whole body of 
water is reduced to the point of greatest density which is 
near the freezing point; hence large bodies like the ocean 
are never frozen in our latitude but the deeper portions 
are always cold ; ( 3 ) part of the heat received on the water 
is expended in producing evaporation and hence does not 
raise the temperature; (4) clouds and fog which check 
radiation of heat are more common over water than over 
land areas. 

Effect of Clouds. — Cloudiness retards loss of heat by 
radiation. In the spring and autumn when the tempera- 
ture is near the freezing point one frequently hears the 
remark, *'If it clears off to-night, there will be a frost." 
On a clear night the heat received from the sun during 
the day is rapidly radiated out into space. If the clouds 
are present this radiation is checked in a large degree, be- 
cause they do not permit the heat to pass through as readily 
as through clear atmosphere and they also radiate and re- 
flect heat back to the earth. 

Prevailing winds affect the temperature in some places. 
In India for about half the year, the winds blow from the 
Indian Ocean warm and moist. During the other half 
they' come from the Thibetan Plateau and Himalaya 
Mountains dry and cold. 

Influence of topography. In high latitudes where the sun's 
rays are quite slanting even at midday, there is great difference 
in the quantity of heat received on the warm south slopes, which 
may be at nearly right angles to the noonday sun, from that on 
the cool north slopes where the rays may be nearly tangent or 
in some cases never strike even at noonday. Slopes facing west 
or southwest are warmer than those facing east or south- 



364 



PHYSICAL GEOGRAPHY 



east. The difference may be manifest in a region moderately 
hilly, but is still more pronounced in a mountainous country, 
especially so where the mountains extend east and west. Are 
there any spots in your vicinity where the spring flowers bloom 
first? Where the early fruits first ripen? What is the relation 
of such spots to the sunshine? (See fig. 249.) 




Fig. 249. Diagram showing different effects of the sunshine on the two 
Bides of a valley. The north side of the valley receives the nearly 
vertical rays of the sun at noonday, while the south side is in the 
shadow. In the winter season this causes more frequent thawing and 
freezing, and hence more rapid weathering on the north side, and in 
the spring it causes earlier vegetation. 

291. Isotherms.— The thermometer is read and re- 
corded twice daily, at 8 o'clock morning and evening, at a 
great many stations in the United States and Canada and 
the results distributed by telegraph to certain places where 
they are recorded on maps and lines drawn connecting 
points having the same temperature ; such lines are called 
isotherms, meaning equal temperatures. By consulting a 
number of daily weather maps for successive days, it will 
be seen that there is considerable variation in the position 
of the isotherms from day to day. These may all be 
averaged ^nd a map constructed showing the mean for 
the month. Fig. 250 shows an isothermal map for July, 
one for January and another for the year. Compare these 
carefully and account for the different positions of the 
same isotherms. 




Fig. 250. Isothermal chart for January, July and for the year. 
Note how the isotherms move southward in the northern win- 
ter, and north in the summer. Why? Why are they deflected 
so much more in North America in the summer and in th© 
north Atlantic in the winter? 



366 PHYSICAL GEOGRAPHY 

292. Temperature Gradient.— On some of the weather maps, 
there are more isotherms than on others, showing at times a 
much greater difference -or range of temperature. In general, 
great differences in temperature are found associated with great 
differences in pressure; in fact extremes of temperature cause 
extremes of pressure. The difference between the high and low 
temperatures is called a temperature gradient and the gradient 
is higher when the isotherms are most numerous and closest to- 
gether. Steep temperature gradients are associated with steep 
or decided pressure gradients. Compare figs. 245 and 248 and 
note that high pressure is associated with low temperature and 
vice versa. The gradients are high in each case. 

293. Temperature Zones.- In the isothermal chart of 
the world, it is shown that the isotherms of 70 degrees lie 
some distance on each side of the equator but at different 
distances in the January and in the July charts. These 
isotherms inclose the warm or hot zone through the midst 
of which runs a line of highest temperature, the heat 
equator, which lies north of the true equator in July and 
south of it in January; that is, it shifts north and south 
following the sun. The temperature belt inclosed between 
the isotherms of 70° and 30° is called the temperate zone, 
while the region around the poles outside of the thirty de- 
gree isotherm has a frigid temperature. All of these zones 
it will be observed shift north and south following the 
movements of the heat equator. (Study fig. 251.) 

All of the temperature zones are more nearly uniform in the 
southern hemisphere than in the northern, and more uniform on 
the southern oceans than on the southern continents. Why? 
The student should be able to state the reasons from the data in 
the preceding pages. 

MOVEMENTS OF THE ATMOSPHERE 

294. Winds, Currents and Calms.— The direct cause 
of the movements of the air is difference in pressure, which 
in turn is due to difference in temperature. Thus, when 



THE ATMOSPHERE 



367 




Fig. 251. Climatic zone map "based on isotherms. Point out some of the varia- 
tions from a zonal map based on the tropical and polar circles. Compare 
with Fig. 250 and note how the zones may shift with the seasons. 



368 PHYSICAL GEOGRAPHY 

the air at any place becomes heated it expands and is 
pushed up by the surrounding air crowding in to take its 
place, which in turn is replaced by air descending at some 
other place. So winds and air currents are produced. 
The movements more or less horizontal, where rapid enough 
to be perceptible, are called winds. The vertical move- 
ments, both up and down, are not ordinarily perceptible 
and are known as calms. The equatorial calms are formed 
by rising currents of air and the tropical calms by descend- 
ing currents, while the movement from the tropical calms 
to the equatorial calms forms the trade winds. 

Classification of the winds. The winds may be grouped 
for classification into terrestrial or planetary, cyclonic or 
eddying, and continental. The wind is named from the 
point of the compass from which it is blowing, hence a 
w^est wind means a movement of the air from the west to- 
ward the east. 

295. Terrestrial or planetary winds are those due to 
the condition of a rotating planet heated from an external 
source, and occur on all planets that have an atmosphere. 
There is an excessive heating in the region of the equator, 
which causes the air to expand and flow off aloft, thus pro- 
ducing a low pressure belt of calms known as the doldrums 
or the equatorial calms and a high pressure near the tropics 
known as the horse latitudes or tropical calms formed by 
descending air. 

Trade winds. The warm air in the doldrums is forced 
upwards by the air which crowds in from the trade winds 
on both sides. The trade winds do not move directly north 
and south to the equator but are deflected to the west be- 
cause of the rotation of the earth, which according to Fer- 
rel's law causes a deflection of all winds, cyclonic as well 
as terrestrial, to their right in the northern hemisphere 
and to their left in the southern hemisphere. It is be- 



V" OF THt 



of 



calii 




VARIABLE 



WINDS 



J. W. REDWAY 



THE ATMOSPHEKE 



369 



cause of the regularity with which these winds blow that 
they are called trade winds. While they are still im- 
portant factors in commerce they were much more so when 
all the vessels were sailing vessels before the days of 
steam navigation. 







winds and Ralna o( July-^Motthftm SottmoC? 

Fig. 253. Planetary wind belts in summer and winter. The heat equator lies 
in the midst of the equatorial rain belt. 
24 



370 PHYSICAL GEOGRAPHY 

Since the doldrum belt shifts north and south following 
the sun during the change of seasons, the trade wind belts 
shift with it. (See fig. 253.) 

The antitrades are caused by the ascending air at the 
equator overflowing out toward the poles in the higher 
atmosphere, above the trade winds and in an opposite di- 
rection. In the vicinity of the tropics, the antitrades 
descend in part to the surface, but since the descent is 
vertical or nearly so, they form belts of calms known as the 
horse latitudes or tropical calms. 

The prevailing westerlies are the winds blowing from 
the horse latitudes towards the poles, but like the trade 
winds and all other atmospheric movements, they follow 
Ferrel's law and are deflected to the right in the northern 
hemisphere and to the left in the southern hemisphere, 
thus becoming northwest winds in the southern hemisphere 
and southwest winds in the northern. They are much less 
regular in their movement than the trade winds, frequently 
shifting direction to take part in the great spiral whirls 
known as cyclones and anticyclones. They are also sub- 
ject to many local disturbances such as land-and-sea breezes 
and the mountain and valley breezes. Beyond the belt of 
the prevailing westerlies, the movements of the atmosphere 
are not so well known; the winds are thought to circle 
around the poles forming the circumpolar whirl or eddy. 

296. Cyclonic Winds.— A second class of winds more 
local and variable than the terrestrial ones are the cyclones, 
in which the winds move inward and upward in a spiral 
whirl or eddy around a region of low barometric pressure. 
In temperate climates, cyclones occur in the belt of the pre- 
vailing westerlies where they cover large areas, sometimes 
1,000 miles or more in diameter. 

There are two movements of the air in a cyclone, a 
horizontal one towards the center and a vertical one at 



THE ATMOSPHERE 



371 



and near the center. The origin of the movement is prob- 
ably due to the increase in temperature at the center which 
causes the air to expand and overflow in the upper atmos- 
phere, producing a downward pressure on the surrounding 




Fig. 254. Cyclone (low) and anticyclone (high) areas in United States. 
Line of arrows show path over which the central low has passed. Shaded 
areas indicate where rain has fallen during preceding 24 hours. Dotted 
lines, isotherms, continuous lines, isobars. Arrows show direction of wind. 
Black circle on the arrow, cloudy, open circle, fair weather. The great 
storm of Feb., 1902 which caused excessive floods in the Ohio valley. 
(U. S. Weather Bureau.) 



area. The crowding towards the center by this downward 
pressure pushes the expanded air up and the movement is 
continued until equilibrium is again restored. The area 
is variously designated a cyclone, a low pressure area or a 
low. It should be noted that the violent whirling storms 
which prove so destructive to life and property, called 
cyclones in the newspapers, are properly called tornadoes, 



372 



PHYSICAL GEOGRAPHY 



and are described below. One or more cyclones pass 
across the United States nearly every week. (Figs. 254 
and 255.) 

An anticyclone is an area of high pressure or a high, 
where the air is descending and the winds blow out along 
the surface from the center. As the name indicates it is 
the opposite of a cyclone, the winds blowing from the anti- 
cyclone or high, to the low. The higher or steeper the 
pressure gradient, that is, the greater the difference be- 
tween the pressure in the center of the low and the high, 
the stronger will be the winds. The air flows down the 
pressure slope at a rate proportional to the steepness of 
the slope. 

The weather in the United States is in a large measure 
determined by the * ' highs ' ' and ' * lows ' ' which move across 
the country in the general direction of the prevailing 
winds. (See sec. 319.) 




Fig. 255. Mean tracks of the high and low pressure areas across the United 
States and the average daily movement of the same. (U. S. Weather 
Bureau.) 



THE ATMOSPHERE 



373 



Movements of cyclones in the United States. Cyclones some- 
times enter the United States from the northwest into Montana 
or Dakota and travel southeast to near the middle of the Missis- 
sippi valley and then northeast to and sometimes across the 
Atlantic Ocean. Sometimes they develop in the southwest in 
New Mexico and Texas and then move northeast off the con- 
tinent. Sometimes they come in from the Pacific Coast. Some- 
times they pass out of the United States in the southeast. The 
rate of advance differs somewhat, but is generally faster in the 
winter, averaging about 800 miles per day, and slower in the 
summer, about 500 miles per day. (See fig. 255.) Test this by 
actual measurement from a series of weather maps for successive 
days. 

297. Hurricanes. — The tropical cyclones or hurricanes 
are great whirling storms from 100 to 300 miles in dia- 
meter, in which the winds frequently become very violent 




Fig. 256. Map showing track of the Galveston hurricane. From the dates 
the rate of movement can be estimated. (U. S. Weather Bureau.) 



374 



PHYSICAL GEOGRAPHY 



and destructive near the center. In the very center, how- 
ever, the winds die away leaving a calm with a clear sky, 
know^n as the ' ' eye of the storm ' ' which is said to vary from 
10 to 20 miles in diameter, while immediately around it 
the winds are most violent, decreasing in intensity further 




Fig. 257. Map showing track ot the Galveston hurricane through a forest in 
Texas. The trees nearly all lie parallel with each other. (W. S. Bray.) 

from the center. The hurricanes originate on the oceans 
within the tropics. The South Atlantic ocean, however, 
appears to be free from them. In the North Atlantic they 
start near the West Indies and are known as West India 
hurricanes. They generally move in a northwesterly direc- 
tion to the coast of Florida and thence northeasterly across 
the Atlantic occasionally reaching the coast of Europe be- 
fore they are dissipated. Sometimes they move up the 
east coast of the United States, causing great destruction 



THE ATMOSPHERE 



375 



to the shipping. Occasionally one of the hurricanes passes 
into the Gulf of Mexico and thence into the Gulf States. 
It was one of these tropical hurricanes that passed over 




Pig. 258. Photograph of a tornado. Note the funnel shaped cloud around 
which the winds move with great velocity. (W. E. Seright. ) 

the city of Galveston in the year 1900 and destroyed the 
greater part of the city, besides doing much damage in the 
country farther north. Similar storms in the Pacific Ocean 



376 PHYSICAL GEOGRAPHY 

are called typhoons and often prove very destructive in 
the region of the Philippines and Japan. (Figs. 256 and 
257.) 

298. Tornadoes are cyclonic whirlwinds of small area 
and great intensity that originate in the region of the pre- 
vailing westerlies. They are associated with thunder- 
storms in the summer season and are most common on 
the plains of the west and southwest. It is not often that 
a violent tornado occurs east of the Alleghany Mountains, 
yet it does sometimes. 

They are generally marked by a dark funnel-shaped 
cloud suspended from the black mass of the thunder cloud 
(see fig. 258). The storm generally moves east or north- 
east at a rate varying from 20 to 40 miles an hour but the 
rotary velocity of the wind in the whirl may reach 500 
miles or more. It is the most violent class of storms known 
in the United States and some of the effects produced are 
almost incredible, such, for instance, as plucking the feath- 
ers from a chicken, tearing the tires from a wagon, tearing 
the lath from a house and driving them through the roof 
of a barn. 

When a tornado occurs on a lake or the ocean, a column of 
water is often formed in the vortex and it is then called a water- 
spout. A vessel caught in one of these waterspouts is liable to 
severe injury if not total destruction. It is thought that the 
water in the waterspoi^t is mostly condensed from the clouds 
rather than drawn up from the sea. 

299. Hot Waves.- The warm, south winds drawn 
northward into a low pressure area, when unseasonably 
warm and dry, are called siroccos. The typical siroccos 
occur in Italy and are caused by the hot, scorching winds 
of the African desert flowing towards a low pressure area 
in central Europe. Similar but not such strongly marked 
siroccos occur at times in the Mississippi Valley, where 



THE ATMOSPHERE 377 

they are known as hot waves and cause drouth in the sum- 
mer season and thaws in the winter. In Australia such 
winds are known as h rick fielders. 

The chinooTc is the warm, drying wind that descends the east- 
ern slope of the Rocky Mountains to the great plains. The 
moisture has been precipitated on the west slope and summit of 
the mountains and the air descends on the plains as dry, hot 
winds. 

300. Cold Waves.— A cold wave signifies a sudden 
fall of the thermometer resulting in temperatures extreme- 
ly low for the season in any given locality. For the winter 
season in central New York a cold wave is defined as a 24 
hour temperature fall of 20 degrees or more to a minimum 
of 10 degrees or lower, while in the warmest portions, of 
the United States a fall of 16 degrees to a minimum of 32 
degrees only is required. It follows a cyclone, precedes 
an anticyclone, and is produced by the cold winds from 
the plains of the west and northwest moving towards a low 
to the east. It is commonly accompanied by fair weather 
but occasionally there is a fine drifting snow and high 
winds forming the much dreaded blizzard of the western 
plains. The Norther of Texas, the huran of Siberia and 
the northeaster of western Europe are local names for a 
cold wave in the different countries. The last is produced 
when a low pressure swings a little farther south than 
usual and the cold winds are drawn down from the plains 
of northern Europe. The northeaster in the United States 
is much dreaded along the Atlantic coast as it is frequently 
the border of an advancing hurricane from the south, 
which means danger to coasting vessels. 

301. Continental Winds.— A third class of winds, 
caused by local differences in the rate of radiation and ab- 
sorption over land and water areas, is called continental 
winds, the most marked type of which is the monsoon. 



378 PHYSICAL GEOGRAPHY 

302. Monsoons.- The monsoons are best developed in 
India where the sea breeze in the summer is so strong as to 
reverse the northern trade winds and cause the southeast 
trades to continue across the equator and over India as 
southwest winds. In the passage across the tropical seas, 
the air is heavily charged with moisture, which is precipi- 
tated on the south slopes of the Himalayas, producing an 
enormously heavy rainfall, in places as high as 35 feet per 
year. In the winter the winds are reversed, the cold winds 
from the plateau of central Asia blowing across India to 
the sea. The winds, warming as they descend the moun- 
tains, blow across India as dry winds, taking up instead 
of precipitating moisture and when they are prolonged they 
produce drouth and famine in the land. The monsoons 
are caused by unequal heating of land and water, the land 
being warmer than water in the summer and cooler in 
winter. (See fig. 252.) 

303. Land and Sea Breezes.- The daily changes be- 
tween land and sea are similar but less pronounced than 
the seasonal. The land is heated during the day, causing 
the air to expand; the inflow from the sea produces the 
sea hreeze during the middle of the day but dies out in the 
night. At night the land cools more rapidly and the 
wind is reversed to the land hreeze blowing from the land 
to the sea during the night and in the early morning. This 
reversal of the winds is utilized by the fishermen who sail 
out on the land breeze in the early morning and return on 
the sea breeze in the evening. 

304. Mountain and Valley Breezes have a similar origin. The 
mountain radiates heat more rapidly at night than the valley, 
hence the heavy and cool air (heavy because cool) flows down 
the mountain and down the valley forming the mountain breezes. 
In the daytime the reverse takes place when the valley hreeze 
blows up the valley and up the mountain. Th« mountain breezes 



THE ATMOSPHERE 



379 




are generally stronger than the valley breezes because in blowing 
down the slopes they are aided by gravity. 
305. Wind Velocity.— The velocity of 
the wind is recorded by an anemometer 
(anemo, wind; meter, measure). The 
style used by the Weather Bureau is 
shown by the accompaning figure. The 
wind blowing into the cups causes them 
to revolve rapidly about the vertical axis, 
the rate of movement being indicated in 
miles per hour by an index at the base of 
the standard. The winds are roughly 
classified according to velocity as follows : 

1. Calm signifies no movement or 
less than one mile per hour. 

2. Light wind, less than 10 miles per 
hour, moves leaves on trees. 

3. Moderate, 10 to 15 miles per hour, moves small branches. 

4. Brisk, 15 to 25 miles, sways branches, raises dust. 

5. High wind, 25 to 40, sways trees. 

6. Gale, 40 to 60, breaks branches, uproots trees. 

7. Hurricane and tornado, above 60, sometimes 500 miles per 
houj", destroys houses. 

The direction of the wind is indicated by a ivind vane which 
consists of an arrow with two broad divergent flanges on the 
opposite end, free to rotate on a vertical axis. The arrow points 
in the direction from which the wind is blowing. The arrows on 
the weather maps point the direction in which the wind is 
blowing. 



Fig, 259. An anemometer 
or wind gauge for meas- 
uring the velocity of the 
wind. 



HUMIDITY AND PRECIPITATION 



306. Absolute and Relative Humidity.— The atmos- 
pliere always carries some moisture in the form of invisible 
water vapor which is obtained by its contact with the sur- 
face of the ocean and the moist land. The amount of 
moisture in the atmosphere varies at ^different places, and 
at the same place at different times. The quantity of 
water in a given volume of air at any time expressed in 



380 PHYSICAL GEOGRAPHY 

grains per cubic foot denotes its absolute humidity. The 
amoimt of vapor present in the air, compared with wHat 
might be present if the air were saturated with moisture, 
gives the relative humidity and is expressed in per cent. 
If the air were perfectly free from moisture, which it never 
is, the relative humidity would be zero; when it is satur- 
ated, the relative humidity is 100 per cent. 

While the absolute humidity may remain constant, the rela- 
tive humidity varies with the temperature. The capacity of the 
air for moisture increases with an increase in temperature. 
Thus the air may be saturated, the relative humidity 100 per 
cent at one temperature, say 60 degrees, and if the temperature 
be raised to 80 degrees the relative humidity will fall consider- 
ably below 100. On the other hand, should the temperature be 
lowered when the relative humidity is 100, precipitation will take 
place, that is, rain or snow will fall. When you hear the expres- 
sions, "The air is raw," "It is penetrating," what can you infer 
concerning the humidity? Why does cold, moist air feel colder 
and warm moist air warmer than dry air at the same tem- 
peratures? 

307. Dew Point.— The temperature at the point of 
saturation is known as the dew point, and may be deter- 
mined experimentally by placing some ice in a cup of 
water and stirring it with a thermometer until moisture 
begins to form on the outside of the cup. The reading of 
the thermometer at that time will be the dew point of the 
atmosphere at that instant. By trying this experiment at 
several different times, it may be noticed that this varies 
considerably in air even at the same temperature. 

308. Instruments.— There are several different instru- 
ments used for measuring the humidity of the air. The 
essential part of a common hygrometer consists of human 
hair deprived of its oil, which changes in length with the 
percentage of moisture in the air. It is called the hair 
hygrometer. 

The sling psychrometer consists of two standard ther- 




THE ATMOSPHERE . 381 

mometers attached to a board, one of which has the bulb 
covered with wet muslin. They are whirled through the 
air for a short time to 
hasten the evaporation 
from the muslin. If the 
air is saturated with 
moisture, the two ther- 
mometers will read the '^^^ 
same, but if the relative 
humidity is low, there 

(will be rapid evapora- Fig. 2b0. Hygrometer. An instrument for 
tion from the muslin ^^^"^f"^ the relative humidity of the at- 

mosphere. A wet and dry thermometer 
covering the wet bulb, is commonly used and the results com- 

causing the mercury to pared. 

fall. The difference between the wet and dry bulb readings 

will increase as the relative humidity decreases. 

A Tiygrodeik is a form of hygrometer in which the result is 
shown directly by adjusting two sliding pieces to the height of 
the mercury in the wet and the dry bulb thermometers, in such 
a way that they control the position of an index which points 
out the number indicating the relative humidity in per cent. 

309. Dew and Frost.— When rapid radiation from 
objects on the surface of the earth causes the temperature 
of the air in contact to be lowered to the point of satura- 
tion, the moisture begins to condense, the point of satura- 
tion being commonly known as the dew point. Dew is 
formed on a clear night by the rapid radiation of the heat 
from the surface after the sun goes down. The air com- 
ing in contact with the cooled and cooling surface is chilled 
by conduction, when some of the moisture condenses as 
dew ; or, if below 32 degrees F., it condenses in the crystal 
form as lioar frost. Dew or frost, is formed most rapidly 
on the surface of substances which are the best radiators of 
heat, such as stone, grass and leaves. 



382 



PHYSICAL GEOGRAPHY 



Less dew is formed on a cloudy than on a clear night because 
the clouds check radiation and prevent the surface from being 
sufficiently cooled. Dew is not formed on a windy night, because 
the air does not remain long enough in contact with the cool 
surface to be lowered to the dew point. 

310. Clouds.— The condensation of the moisture in the 
air produces clouds of many different shapes and sizes. 
A cloud at the surface of the earth is called fog, or, if very 
light, mist. In the fog or cloud, the moisture has been suf- 
ficiently condensed to form small particles large enough to 
intercept the rays of light. 

311. Classification of Clouds.— The more common 
forms of clouds are: (1) the cumulus which often resem- 




FlG. 261 Cumulus cloud. 
cedes a thunder storm. 



Common in the sumimsr scasou. IVequeutly pru 



bles great masses of snowy wool or cotton. They com- 
monly have a flat or nearly regular base but a very irre- 
gular and changing top and are among the most common 
cloud forms in the summer season. They are formed by 
the ascending currents of warm air from the heated land 
surface. They are 'also called thunderheads and commonly 



THE ATMOSPHERE 383 

precede a thunderstorm. The base is generally half a 
mile to a mile above the, surface of the earth. 




Fig. 262. Plumed cirrus cloud. Height 8000 meters in winter, 9700 meters 
summer. (E. E. Howell.) 



384 



PHYSICAL GEOGRAPHY 



(2) The cirrus cloud is a feathery, plume-like form 
that occurs at a height of five to ten miles above the surface 
and often consists of fine ice or snow crystals, owing to its 
great height. It forms in the front of an advancing cy- 
clone or low pressure area and, moving ahead with the 
*'low," is a pretty good indication of the advance of a 
storm center. It has been called a ' ' weather-breedellr " be- 
cause it is frequently followed by rain or snow. 

(3) Stratus clouds occur in layers or strata near the 
surface and frequently accompany rainstorms. They 
sometimes fall to the surface and form fogs. They are 
common in the early morning, but may occur at other 
hours. 




Tic. 263. Cirro cumulus cloud. Height 6500 meters summer. (E. 
HoweU.) 



(4) Nimbus is a rain cloud, consisting of a dark grey 
to black mass generally covering the whole sky and from 
which the rain falls. (The term ^^nimhus^' refers to a 



THE ATMOSPHERE 385 

condition rather than to a form of cloud and is usually 
understood to be any mass of cloud from which rain or 
snow is falling.) Any of the other clouds, especially the 
stratus or cumulus, may rapidly change to a nimbus. This 
is the most common cloud form in New York State during 
the winter, lasting at times for several weeks with the rain 
falling at intervals! 

The different cloud forms mentioned may form many 
combinations, as cirro-cumulus j cirro-stratus, cumulo-stra- 
tus, strato-cumulus. 

312. Precipitation.— i?am occurs when the moisture 
condenses into drops which fail to the earth. If the 
condensation takes place at temperature below 32 degrees 
F. it forms snow, which bears the same relation to the rain 
in the clouds that frost does to dew on the surface of the 
earth. The moisture may condense as snow in the higher 
air and, in falling through warmer currents near the sur- 
face, may melt and reach the surface as rain. But if the 
rain should freeze while falling through the lower air it 
would not form snow but sleet. Sleet may also be half- 
melted snow. 

Hail is thought to be a mixture of snow and frozen rain. 
It is formed during thunder- 
storms in the summer season 
probably by the passage of 
the descending moisture through 
several air currents with tem- 
peratures alternately above and 
below the freezing point. Hail 
storms, coming as they do in the 
summer season often cause great 
damage to vegetation. 

o-tn ^ M.M A w« . m.. -ciw. i4D4. iippzng ram gauge. 

313. Ouantitv of Rain. — The >r ^-^ & ^ * 

vs"«'"*'-i«'j' "A ivrt'xxi. xut; Measures and records auto- 

amount of rainfall is determined maticaily the rainfall. 
25 




386 



PHYSICAL GEOGRAPHY 



by measuring the depth of water in a vessel known as a rain- 
gauge. A section of the rain-gauge used by the U. S. Weather 
Bureau is shown in fig. 264. Snow and hail are melted and the 
result given in the amount of water as though it had fallen as 
rain. It takes about 8 or 10 inches of snow to equal an inch of 
rain, but this differs with the kind of snow. 

Most all the rainfall may be included under the three 
heads: cyclonic, tropical and monsoon. 

314. Cyclonic Rains.— In the region of the prevail- 
ing westerlies, most of the rainfall comes from the cyclonic 
or low pressure areas. As the cyclone moves east across 
the country, the warm, moist winds, drawn in from the 
south and east, ascend in the atmospheric whirl and are 



189° 125° 121° n7« iiy iQg" 106° ipi" »r 93° 88° g5° 81° 77° 73° W 06° 




Fig. 265. Rainfall map of the United States showing the mean annual rainfall 
in different portions of the country. Give your reasons for the great vari- 
ation. More of the zonal lines run north and south than east and west. 
Account for the maximum and minimum in the same latitude. (U. S. 
Weather Bureau.) 



cooled as they rise; the moisture they carry is condensed 
and falls as rain or snow. The greater part of the rain 



THE ATMOSPHERE . 387 

falls to the east or the southeast of the cyclone center. 
Verify this by study of the weather maps. 

Thunderstorms. In the summer season the cyclones 
are frequently accompanied by thunderstorms which are 
most frequent to the east and south of the cyclone center, 
but are not limited to these parts. They are produced by 
rapidly ascending warm air currents which produce a 
heavy cumulus cloud, the downward pressure of which 
causes reversed air currents to spread out on the surface 
in the midst of the ascending warm currents in front of 
the rapidly moving clouds. The outrushing blast of cool, 
refreshing air is generally followed closely by a downpour 
of rain which may continue for a few minutes or for sev- 
eral hours. Thunderstorms are most frequent in the latter 
part of the afternoon or night. The lightning is caused 
by the passage of the electric spark from cloud to cloud or 
between the earth and the cloud. Thunder is the sound 
caused by the violent agitation of the air along the flash. 
Since the velocity of light is nearly instantaneous for 
short distances and sound travels about twelve miles per 
minute, the distance of a lightning flash may be roughly 
estimated in miles by dividing by five the number of sec- 
onds that elapse between the flash of lightning and the 
sound of the thunder. Much of the rain in the summer 
season in the Mississippi Valley comes from the thunder- 
storms. 

Cloudbursts associated with thunderstorms and torna- 
does are thought to be caused by ascending air currents so 
violent that they hold up the condensed moisture for some 
time, until the accumulation Anally breaks through and 
the water falls in a mass or sheet, frequently causing dis- 
aster on the surface where it falls. Cloudbursts are most 
frequent in dry or semi-arid regions, often proving destruc- 
tive in the mountains of Arizona, New Mexico and Col- 



388 , PHYSICAL GEOGRAPHY 

orado. Many persons have been drowned from the cloud- 
bursts in these states and on the Sahara and other deserts. 
The tropical rains in the doldrum belt are of almost 
daily occurrence. The clouds begin to form near the mid- 
dle of the day and heavy rains, generally accompanied by 
thunderstorms, follow in the early afternoon. The sky 
clears at night and the morning is fair. These rains con- 
tinue throughout the year shifting north and south, fol- 
lowing the movements of the heat equator. (See fig. 253.) 

WEATHER AND CLIMATE 

315. Weather refers to all the atmospheric conditions 
that can be seen or felt, such as (1) the temperature, 
whether hot or cold, or growing warmer or colder ; ( 2 ) pre- 
cipitation, whether rain or snow and how much ; ( 3 ) Cloud- 
iness, whether fair, partly or wholly cloudy, and the rela- 
tive humidity of the air; (4) winds, direction and velocity, 
and changes. 

316. Climate is the sum total of the average weather 
conditions for a series of years; its consideration should 
include also a statement of the extremes or variations from 
the normal, and would have the same elements of temper- 
ature, moisture and winds as the weather. The whole area 
of the earth is sometimes divided into five climatic zones, 
separated by certain parallels of latitude, but if all the 
elements of climate are considered, the zones would be more 
irregular and might have a number of subdivisions such as 
those indicated in fig. 251. 

The controlling factors of the climate are: 
(1) Latitude. Outside of the tropics the sun's rays 
become more and more inclined as one advances towards 
the poles and the climate becomes correspondingly cooler, 
hence the division into torrid, temperate and frigid cli- 
mates. 



THE ATMOSPHERE 389 

(2) Altitude. Since the density of the air and hence 
the temperature decreases with the altitude, cooler climates 
will be found in ascending the mountains and plateaus. 

(3) Distance from the ocean or other large body of 
water affects the uniformity of temperature and frequently 
the moisture or precipitation. 

317. Effect of Mountains on Climate.— The relation of moun- 
tains to an area often has a marked effect on its climate, an 
effect most pronounced in the region of prevailing winds, where 
they blow across the mountains. In the trade wind belt in South' 
America, there is a heavy rainfall on the east or windward side 
of the Andes Mountains, while on the lee side there is a very 
light rainfall. In southern India in the monsoon belt, there are 
heavy rains during the part of the year when the summer mon- 
soons are blowing, while the remainder of the year is dry. 

318. The terrestrial wind belts are factors of prime impor- 
tance in determining the climate. The doldrum belt has its al- 
most daily rains and uniformly moist climate. The trade wind 
belt has a generally uniform fair weather on the seas and fre- 
quent rains on the lands, where the winds blow over considerable 
elevations, but dry on the lee side of the mountains. Most of the 
deserts occur in this belt. The subtropical belt over which the 
horse latitudes migrate have prevailingly fair weather, but an 
area lying in this belt is swept by the trade winds in one season 
and by the prevailing westerlies at another. It is an area of dry 
summers and rainy winters. (See fig. 251.) 

319. The cyclone and anticyclone paths are controlling 
factors of the weather in the belt of the prevailing wester- 
lies, and determine the weather changes from day to day. 
These should be studied carefully from the daily weather 
maps published by the government, and should be studied 
on groups of maps for several different months in differ- 
ent seasons. 

The following weather conditions* may be looked for 
in association with the lows and highs in the United States : 

"High winds with rain or snow usually precede the low. In 
advance of the low the winds are generally southerly and con- 
* Quoted from the Chief of the Weather Bureau. 



390 PHYSICAL GEOGRAPHY 

sequently bring high temperatures. When the center of the low 
passes to the east of a place, the wind at once shifts to the west 
or northwest, bringing low temperature. The temperature on a 
given parallel west of a low may be reasonably looked for on the 
same parallel to the east when the low has passed. Frost will 
occur along the north of an isotherm of about 40 if the night is 
clear and there is little wind. Following the low usually comes 
an area of high, bringing sunshiny weather, which in turn is fol- 
lowed by anoth r low. 

"The cloud and rain area in front of a low is generally about 
the size of the latter and oval, with the west side touching the 
center of the low in advance of which it progresses. 

"When the isotherms run nearly east and west no decided 
changes in temperature will occur. If the isotherms directly west 
of a place incline northwest to southeast it will be warmer; 
if from northeast to southwest it will be colder. 

"An absence of decided waves of high or troughs of low 
pressure indicates a continuance of existing weather, which will 
last until later maps show change, usually first appearing in the 
west." 

320. Weather Maps.— At eight* o'clock each morn- 
ing and evening at many places in the United States, 
Canada, Mexico and the West Indies, observations are 
made on the weather conditions. A record is made of 
the barometric pressure, temperature, velocity and direc- 
tion of the wind, condition of the sky, relative humidity, 
and amount of precipitation, and within the hour these 
data in a condensed cipher dispatch are sent by telegraph 
to the Weather Bureau in Washington. The data are 
rapidly tabulated and transferred by appropriate symbols 
to a weather map, which is in turn engraved and a large 
edition printed and delivered to the mails all within a 
remarkably short period of time. Smaller editions of a 
less elaborate map are printed at the local stations in dif- 
ferent cities outside of Washington. 

*Eight o'clock by the 75th meridian time, which means 7 o'clock at St. 
Louis, 6 o'clock at Denver, and 5 o'clock at San Francisco. 



THE ATMOSPHERE 391 

The data shown on the weather map consist of (1) iso- 
bars, represented by solid black lines drawn through points 
having the !l ame atmospheric pressure, a line for each tenth 
of an inch on the barometer; these lines curve around and 
enclose the lows and the highs. (2) red lines (on the local 
maps, dotted lines) are drawn for the isotherms, one for 
each 10 degrees difference in temperature. (3) Heavy, 
broken red lines enclose areas where there has been a de- 
cided change in temperature equal to a rise or fall of 20 
degrees or more in 24 hours. (4) Shaded areas (on the 
"Washington map but not shown on the local map) indi- 
cate the area over which there has been rain or snow dur- 
ing the past 24 hours. (5) The condition of the sky is 
indicated by a small circle, which is black for cloudy sky 
and open for clear sky. The arrow on the circle indicates 
the direction of the wind. R signifies rain and S snow. 

Besides the graphic representation by lines and sym- 
bols, all the data are printed in tabulated form on the 
margin of the map. Daily weather maps from the nearest 
Weather Bureau office or from Washington can generally 
be secured on application and should be studied along with 
the text. 

Many thousands of the weather ma^^o are distributed daily 
through the mails and by messengers. Besides the maps there 
are thousands of cards sent out from the local stations which 
contain simply the weather forecasts for the next 24 hours. 
These cards are sometimes distributed in large numbers by busi- 
ness firms and are displayed in stores, post offices, railway sta- 
tions, elevators, and other public places. In some places the 
rural mail carriers display weather signals on the mail carts and 
in places the signals are displayed on some prominent point as 
a steeple, flagpole, or some tall building. 

The flag signals are as follows: A square white flag for fair 
weather; a square blue flag for rain or snow; a triangular black 
flag above the white or blue flag indicates followed by warmer 
weather; if below, by colder; a square white flag with a square 




Fig. 266. Daily weather map. Note the movements of the lows and highs 
for the two days. On February 3rd, the intervening day, the one low 
was central over southern Maine, the other over the Pan Handle of Texas. 
(U. S. Weather Bureau.) 



THE ATMOSPHERE 393 

black center indicates a cold wave coming. There is another set 
of flag signals in use for wind storms on the lakes or sea shore. 

321. Benefits from Weather Forecasts.— Some of the 
many benefits that may be derived from the widespread 
distribution and heralding of the weather forecasts are sug- 
gested in the following : Knowledge of a tropical hurricane 
in the West Indies arrives by cable and storm signals are 
placed in all the harbors along the Atlantic coast from 24 
to 36 hours ahead of its arrival, by which many vessels are 
saved from destruction. Similar forecasts of storms save 
a great many boats on the Great Lakes. Some of the in- 
surance companies recognize the value of this branch of 
the service by refusing all risks on vessels that go out 
against the warnings. 

The news of a decided cold wave coming from the northwest 
causes quite a flutter in many lines of business, the ice com- 
panies, the coal dealers, the railway employees in charge of 
perishable goods, the fruit commission merchants, stock raisers, 
and many others who take such precautions as they can to pre- 
vent loss. An important branch of the service consists in the 
warnings of floods along the larger rivers in which the fore- 
knowledge is often the means of saving a great deal of property. 

The student may enumerate other ways in which benefit may 
be derived from the foreknowledge of the weather changes. 

The weather forecast is given for 24, sometimes 48 hours 
ahead. There has been considerable study in trying to find some 
scientific basis of foretelling the weather conditions some weeks 
or months ahead, but no definite results have been obtained. 
The weather conditions published in certain pamphlets and 
almanacs for the entire year have little if any scientific value. 

322. Climatic Zones.— The surface of the globe is 
commonly divided into five climatic zones, based on an 
arbitrary division of so many degrees of latitude. Thus 
the torrid zone includes all the area between the tropics, 
the two temperate zones the areas between the tropics and 
the polar circles, while the remainder is in the frigid zones. 



394 PHYSICAL GEOGRAPHY 

It has been shown, however, that the unequal distribution 
of land and water causes a distribution of winds, rains, 
and temperature that does not follow the parallels. In 
comparing the isothermal chart of the world for the year 
and for the winter and summer seasons it will be seen that 
the temperature inside of the tropics in one place is quite 
different from that in the tropics in another place. A 
comparison of the rainfall in different areas shows even 
more marked differences. (See sec. 293, fig. 251.) 

A more practical division of the surface into tempera- 
ture zones would be based on isotherms rather than on 
parallels. Some of the rather well defined climatic types 
that occur in different areas are (1) the doldrums of the 
tropics with warm, moist climate and persistent rainfall ; 
(2) the trade wind belt which is warm and wet on the 
east side of the continents and generally dry, sometimes a 
desert, on the west side of the continent ; ( 3 ) the monsoon 
belt with the wet and dry seasons; (4) the subtropical 
belts over which the dry tropical calms, the frequently pre- 
cipitating trade winds and the prevailing westerlies mi- 
grate at different seasons. The temperate zone may be 
divided into two parts, (5) that nearer the tropics char- 
acterized by warm summers and mild winters, and (6) 
the outer portions, by hot summers and cold winters. 

There is a marked difference between the climate on 
the seashore and that of the interior, between the eastern 
and western shores of both the continents and the oceans. 
Likewise between the plain, plateau, and mountain cli- 
mates. Find some examples of each. 

323. Changes in Climate.— One frequently hears the 
statement that the climate is changing— that there is not 
so much snow and that the winters are not so cold as they 
used to be. Such remarks apply to the weather rather 
than to the climate. There are frequently quite marked 



THE ATMOSPHERE 395 

changes between successive seasons, but the official weather 
records do not indicate any marked changes in the climate 
back as far as the record has been kept. 

324. Geological Climates.— The geological record which ex- 
tends over millions instead of a few tens of years, shows many 
pronounced climatic changes. For instance, some few thousand 
years ago the climate was enough colder than that at present in 
the northern hemisphere, to cause an accumulation of snow and 
ice in the form of great glaciers over all the north central parts 
of North America and Europe. This condition continued ap- 
parently for thousands of years. In a preceding geological period 
it was warm enough for the growth of tropical plants as far 
north as the Arctic Circle. 

In still earlier geological times, a very long time ago, the 
climate of central New York and southern Michigan was ex- 
ceedingly dry, possibly as dry as that of Utah to-day. This is 
shown in the record by the great beds of rock salt in this region. 

325. Electric and Optical Phenomena.— Lightning is 
caused by the electric discharge in the form of a vivid flash 
or spark between clouds or between a cloud and the earth. 
The lightning is associated with a heated atmosphere and 
is common in the hot summer season but is absent in the 
winter season, except rarely when the air becomes unsea- 
sonably warm. It appears also to be associated with move- 
ments of the warm air currents and hence accompanies 
the violent air movements of thunderstorms and tornadoes. 
Probably the moisture in the cloud is also an important 
element in the electric discharge. The energy, or electro- 
motive force, manifest in a violent thunderstorm, is far in 
excess of that produced by any artificial means. It takes 
several different forms, known as zig-zag or chain light- 
ning, heat lightning and sheet lightning. 

The passage of the electric current from the earth to the 
cloud, or the cloud to the earth, is likely to be from some ele- 
vated point as a tree, a church steeple, or some tall building, 
yet this is not always the case as lightning has been known to 



396 



PHYSICAL GEOGRAPHY 



strike animals and other objects in the near vicinity of trees 
and buildings without injury to the tall object. (Fig. 267.) 

The thunder is caused by the inrushing air to fill the partial 
vacuum produced by the lightning flash. The interval of time 
between the flash of the lightning and the sound of the thunder 
is an indication of the distance of the flash. 




Fig. 267. Lightning flash, Lincoln, Neb. (U, G. Cornell.) 

326. St. Elmo's Fire is a brush discharge of electric- 
ity often observed during electric storms on steeples, 
masts of vessels at sea, and other high points. It is un- 
accompanied by the noise or danger of the lightning flash. 

327. The aurora horealis is presumably an electric 
phenomenon, the cause of which has not been satisfactorily 
explained. Since in the northern hemisphere it is always 
observed in the north it is commonly called the northern 
lights. It is occasionally observed as far south as New 
York, but it is much more frequent and spectacular in the 
higher latitudes where it is an object of much interest dur- 



THE ATMOSPHERE 397 

ing the long northern winter. It consists of a great arch 
or sheets of light stretched across the northern sky, from 
which great streamers of many and fantastic forms ex- 
tend to, or towards, the zenith. It possibly has some rela- 
tion to the magnetic poles of the earth. 

328. The rainbow is an arch of prismatic color that is 
produced by the refraction and reflection of the light from 
the interior of the raindrops ; the light emerging from the 
drop is separated into the prismatic colors. Frequently 
a second bow appears and even a third and fourth have 
been reported. A rainbow usually shows less than half a 
circle. Under what conditions does it appear a half circle ? 
If one could see the rainbow from a balloon how much of 
the circle would appear ? 

329. Coronas or rings around the moon, sun dogs, moon dogs, 
and halos are other phenomena due to refraction and reflection 
of the light in the upper atmosphere, sometimes from the little 
ice or snow crystals, sometimes from drops of moisture. Cor- 
onas are due to diffraction and interference. Halos are caused 
by reflection and refraction of the light in the small ice crystals. 

The different colors of the sky are due to refraction and 
selective scattering of the different prismatic rays. When there 
is much dust in the atmosphere the bright red and yellow colors 
are reflected to the eye at sunrise and sunset. 

330. The mii-age is caused by the turning of the rays 
of light from their original direction, causing objects to 
appear to be out of place. It is produced by the atmos- 
phere occurring at times in layers of different density. The 
light rays which have already been bent from their original 
course are reflected to the eye from the surface of one of 
the layers, causing the object to appear out of position and 
frequently out of proportion. 

The desert mirage occurs over hot, dry land areas, by the 
reflection from the layers near the earth to the eye, giving the 
appearance of the reflection of trees from a smooth water sur- 
face. Many a person has been lured to destruction by following 



398 PHYSICAL GEOGRAPHY 

these phantom lakes across the scorching sands of the desert. 
The stratification of the lower layers of air is due to the intense 
heating of the air near the ground, which causes it to expand, 
but the air being quiet, there accumulates considerable pressure 
before convectional currents are started. Sometimes the flight 
of a bird is sufficient to disturb this unstable equilibrium and 
start the uprush of heated air which frequently produces a whirl- 
wind. Sometimes a half dozen or more of these whirlwinds are 
visible at one time on the sandy plains of the desert on a hot 
dummer day. 

The mirage is sometimes visible on the sea, where the reflec- 
tion is from the upper atmosphere down to the eye of the ob- 
server, causing ships and other objects below the horizon to 
appear in the sky sometimes upright and sometimes inverted. 
This form of mirage is known as the 



331. The zodiacal light is a disk of faint light sur- 
rounding the sun. It may be seen as a triangular column 
of light, rising from the western horizon shortly after twi- 
light in the winter and spring and in the east before day- 
break from September to January. It is thought to be 
sunlight reflected from a cloud of meteorites revolving 
around the sun. Another theory for the zodiacal light is 
that it is caused by particles electrically discharged from 
the poles of the sun and condensed along the plane of its 
equator. 

332. The Gegenschein (counter-glow) is a faint patch of light 
on the ecliptic directly opposite the sun. One hypothesis for its 
occurrence is that it is caused by meteors which tend to con- 
dense directly opposite the sun. Another explanation is that it 
forms a tail to the earth similar to the tail of a comet composed 
of particles of helium and hydrogen escaping from the earth. 

REFERENCES 

1. Davis, Elementary Meteorology. Ginn & Co., Boston, 1894. 

2. Waldo, Modern Meteorology, Scribner's Sons, New York, 

1893. 

3. Ward, Practical Exercises in Elementary Meteorology, 

Ginn & Co., Boston, 1896. 



THE ATMOSPHERE 399 

4. Ferrel, Popular Treatise on the Winds, Wiley & SonS; 

New York, 1889. 

5. Annual Reports, Monthly Weather Review and Daily 

Weather Map by the U. S. Weather Bureau, Wash- 
ington, D. C. 

6. Harrington, Rainfall and Snowfall of the United States, 

Bulletin C, U. S. Weather Bureau, Washington, D. C. 

7. Greely, American Weather, Dodd, Mead & Co., New York, 

1903. 

8. Ward, Hann's Handbook of Climatology, MacMillan & Co., 

New York, 1903. 

9. Harrington, Weather Making, Ancient and Modern, An. 

Rept. Smithsonian Institution, 1894, pp. 249-271. 

10. Davis, Practical Exercises in Geography, Nat'l Geog. Mag. 

Vol. XI, p. 62. 

11. Garriott, West Indian Hurricanes, Nat'l Geog. Mag., Vol. 

X, p. 343, and Vol. XI, p. 384. 



CHAPTER XI 
GEOGRAPHY OF PLANTS, ANIMALS, AND MAN 

333. Influence of Environment on Life.— All forms 
of life are of necessity influenced by their physical en- 
vironment. The kind, the abundance, the variety of living 
forms on any area largely depends upon the geographical 
conditions of soil, climate, and topography on the land ; and 
temperature, depth, and clearness of the waters in the 
sea. This has been suggested from time to time in the pre- 
ceding chapters, but it seems fitting now in conclusion to 
consider the subject directly in reference to the life rela- 
tions. 

Man is probably as nearly independent of his geo- 
graphical surroundings as any other form of life, but that 
in his migrations, civilization, industries, and mental as 
well as physical development, he has been greatly influ- 
enced by geographical conditions is apparent to all. Many 
of the lower forms of life are more susceptible than man 
to their surroundings and hence occupy only a few limited 
areas, while man ranges over the earth from the equator 
nearly to the pole and is making strenuous efl'orts to reach 
that hitherto inaccessible point. 

334. Effect of Climate.— There are striking differ- 
ehces in the kinds of life and the habits of the living forms 
in the different climatic zones. In the warm, humid 
region, life, death, and decay go on with striking uniform- 
ity and rapidity throughout the year and the years. In 
the cold temperate zones there is a warm season of rapid 
growth, and a cold season of rest, when the trees and shrubs 

400 



n 



GEOGRAPHY OF LIFE 



401 



shed their leaves and fruit, and the herbs and grasses die 
and disappear all but the roots, bulbs, and seeds. Many 
of the animals hibernate. Man, the domestic animals, and 
some of the wild animals remain active during the cold 
weather of the winter months, but the lower forms of 
life,— many of the animals, and all the vegetable forms— 
lie dormant and inactive until the return of warm weather. 
The winter in cold climates is characteristically a sea- 
son of silence. At a distance from human habitations al- 
most the only sounds are those of inanimate nature. 

"With the coming ot the spring there is a marvellous awak- 
ening and unfolding. The brooks, swollen to overflowing by the 




Fig. 268. Winter in cold climates is a season Oi silence. Winter scene in an 
evergreen forest in the United States. 



melting of the snow, make music as they run. The northward 
flight of the birds brings to every grove a chorus of song. A 
host of batrachians and reptiles bestir themselves after a long 
26 



402 PHYSICAL GEOGRAPHY 

winter sleep and vociferously proclaim their presence. The in- 
sect world, with its unnumbered legions, takes wing. The air 
vibrates with millions of voices. The trees put forth their leaves, 
each a harp-string which responds to the touch of the fingers of 
the wind. The organ-notes of the thunder again startle the hiber- 
nating echoes. As the winter is the silent season, so the spring 
is the time of music." (Russell's North America, pp. 296-297.) 

Make a list of the birds and wild animals you see or 
know to be alive in our fields and forests in the winter. 
Note the date when you see the first birds in the spring 
and the ones that come first. What animals hibernate or 
sleep during the cold season? (See fig. 268.) 

This seasonal renewal of the activities of the varied 
forms of life is probably one of the reasons why man has 
made his greatest advancement in the temperate zones. 

PLANT GEOGRAPHY 

The number and kinds of plants on any area not under 
cultivation are determined largely by the condition of the 
soil, w^ater, air, and temperature. 

335. Soil. — Most land plants have roots which find 
anchorage in the soil from which they derive sustenance 
both in water and mineral matter. While but a small part 
of the plant is formed by the mineral matter in the soil, 
that small part is so important that if the materials are 
not in the soil the vegetation does not flourish. Thus a 
grain of wheat that would sprout and grow a spindly stalk 
a foot high with no grains on poor soil, would grow a lusty 
stalk four feet high, with many good grains, on a fertile 
soil. 

The kind of soil has much to do with the variety and quan- 
tity of vegetation. Thus the vegetation on a sand soil will be 
different from that on a rock, clay, loam, humus, or alkaline soil. 
Much depends on the relation of these soils to each other; thus 
a humus on sand would be different from a humus on clay. More 



GEOGRAPHY OF LIFE 



403 



Important than the chemical proportions are the physical ones, 
such as, the fineness of the particles and the porosity of the mass 
as affecting the circulation, absorption, and retention of moisture. 
Some forms of life are independent of the soil, such as floating 
vegetation, which derives sustenance, wholly from the water and 
air. A few land plants live without contact with the soil and 




jjiG, 269. Spanish moss on live oak tree, Columbia, 
A common epiphyte in the southern United States. 



(W. L. Bray.) 



derive sustenance wholly from the air. Such are called epiphytes, 
because they grow upon the stems and branches of other plants. 
They are most abundant in the tropics. Fig, 269 shows the 
Spanish moss an epiphyte that grows in great abundance in the 
southern and southwestern United States. The mistletoe is a 
common epiphyte widely distributed through the United States 
and Europe. 

336. Water.— Water forms a large part of the material 

of nearly all plants and hence is essential to their existence. 
The kind of water, whether fresh, salt, or alkaline, and the 
quantity of it, whether an excess as in the marsh, a dearth 



404 



PHYSICAL GEOGKAPHY 



as in the desert, a limited supply as in the semi-arid dis- 
tricts, or a generous amount as in the humid districts ; and 
the temperature, whether hot, temperate, or cold, determine 
in a large degree the kind as well as the quantity of the 
vegetation. The distribution of the rainfall throughout 
the year is important; whether it falls in the winter or in 
the growing season. The relation of the water table to 
the surface soil is also an important factor. 




Fig. 270. Some types of water plants growing in a swamp of northern 
United States. Some float on the surface, others extend several feet above 
the surface, while others grow in the bottom entirely under water. View 
in the Montezuma Swamp, N. Y. (E. R. Smith.) 

337. Classification of Plants Based on Water Supply. 
— On a basis of humidity plants may be divided into (1) 
water plants, (Hydrophytes) ; (2) drouth or desert plants 
(Xerophytes) ; (3) intermediates (Mesophytes). 

1. Some water plants grow on the surface of the water, 



GEOGRAPHY OF LIFE 



405 



others on the bottom. Another class includes those with 
roots on the bottom and leaves and branches above the sur- 




FlG. 271. Illustrating the function of cypress knees. A, level of water in 
the growing season. B, lowest level of swamp water; a, cypress tree with 
part of the roots under water; b, tree with all roots under water; c, tree 
with none of the roots under water; dd, cypress knees through which the 
roots breathe; ee, knees not yet gi'own to serviceable height; ff, abortive 
knees not needed by the tree. (After Shaler.) 




Fig. 272. Cypress trees and knees in the Great Dismal Swamp of Virginia. 
The knees are protuberances on the roots extending sometimes to several 
feet above the surface of the water. (U. S. Geol. Survey.) 

face, such as the mangrove tree in Florida, water lilies, cat- 
tails, splatter dock, reeds, and cane in the lakes and 
swamps. The water hyacinth is a floating plant that grows 



OF THE 

UNIVfiRSITY 



406 



PHYSICAL GEOGRAPHY 




Via. 273. Papago Indian, Sonora, Mexico, crushing the pulp of the interior 
of a barrel cactus in order to squeeze the water out of it. Sometimes a 
gallon or more of water is obtained in this way from a single plant. 
(D. T. McDougal.) 



GEOGRAPHY OF LIFE 407 

in such quantities on the rivers and lakes of Florida as to 
be a serious menace to navigation and the fishing industries 
on the inland waters. (Fig. 270.) 

Cypress trees that grow sometimes in the water and some- 
times on dry land develop a unique method of getting air to the 
roots that grow under water, by having a knob or process grow 
up through the water to and above the surface until it is in con- 
tact with the air. These knobs which are known as "knees," 
stand above the surface, looking much like stumps. (See figs. 
271 and 272.) After the drainage of the swamp the new growth 
of cypress on the dry land is devoid of knees. 

The Sargassum that occurs so abundantly out in mid-ocean 
has numerous small air sacs which serve as floats or life-pre- 
servers to keep it at the surface. The Bladderroot, a fresh water 
plant, has similar floats. The bladders or air sacs serve to 
aerate the plant as well as float it. The duckweed, a small green 
floating plant has numerous air chambers through the body of 
the plant. There is another class of water plants, microscopic 
in size, that occurs in vast quantities in both salt and fresh water. 
The best known forms in this class are the diatoms which 
secrete a wall of silica and hence are preserved in great deposits 
in the bottom of the sea and lakes. (See sec. 105.) 

2. Desert plants which have the opposite condition 
from the water plants, have many ways for collecting, re- 
taining and conserving the moisture and prolonging their 
existence in the absence of rain. The scarcity of leaves 
is one device, as it is from the leaves that evaporation 
takes place. Some of the cacti have no leaves. A hairy 
covering over the leaves in some plants serves to shade thera 
from the rays of the sun. Thorns and brambles serve to 
protect many of them from being eaten by animals. The 
cactus, sagebrush, greasewood and yucca are among the 
most common plants on our western deserts. The semi- 
arid regions, which are subject to drought at regular or ir- 
regular periods, have a greater variety and number of 
plants than the desert. In such areas some of the plants 
store water in reservoirs provided for that purpose in 



408 



PHYSICAL GEOGRAPHY 




GEOGRAPHY OF LIFE 



409 



the leaves and stems. Others conserve moisture by curling 
the leaves and thus exposing a smaller surface. (Figs. 
273, 274, and 275.) 




Fig. 275. Oasis of palms {Neo Washingtonia filifera) in the Colorado Desert, 
near Indio, Cal. (See FiG. 79.) Any part of a desert area with water at 
or near the surface is an Oasis. It is a green spot in the midst of the 
brown waste. (D. T. McDougal.) 

3. The intermediate plants that grow on dry land sup- 
ported by a fairly abundant rainfall are the most numerous 
and comprise about 80 per cent of the total flora. They 
are called mesophytes because they occur midway between 
the very dry and the very wet conditions. They include 
most of our cultivated plants and most of the forest trees. 
They occur in the same rain belts as the water plants but 
under different physiographic conditions, that is, where 
there is sufficient rainfall but the water does not stand on 
the surface as in the case of the hydrophytes. 

They occur in different rain belts from the desert plants. 
The dividing line is about 15 inches annual rainfall. It varies 



410 



PHYSICAL GEOGRAPHY 



considerably with the distribution of the rainfall and other con- 
ditions. It requires about 20 inches of annual rainfall to support 
forest growth, but under certain conditions some coniferous trees 
exist in cold climates on less than that. 

The slopes of the Rocky Mountains are covered with forests 
(where they have not been destroyed) which form a belt between 




Fig. 276. View at the timber line in the Rocky Mountains, Ouray County, 
Colo. The upper portion of the mountains are void of trees. The forests 
are dense towards the base of the mountain. Snow lies on the high moun- 
tains nearly all the year. 



the plains at the base, treeless from lack of rainfall, and the tree- 
less peaks at the top made so from excess of snow. On all very 
high mountains even in the tropics there is an upper limit to 
tree growth known as the timber line. (See fig. 276.) 

The upper limit of trees is not due directly to cold but to 
excess of snow, as shown by the occurrence of trees on the nar- 
row ridges much higher than in the depressions. It is in the 
depressions where the snow accumulates and remains long after 
it has melted from the ridges, so that the growing season when 
the ground is free from snow is too short for trees to develop. 

In respect to their association, there are two great groups of 
the intermediate plants that have a very pronounced effect on 



GEOGRAPHY OF LIFE 



411 



the surface. The one group consists of grasses and Jierhs and 
forms the meadows, prairies, pastures, and tundras. The other 
consists of shrubs and trees and forms the thickets and forests. 
With change of conditions each of these may encroach upon the 
territory of the other. Fig. 277 shows an area in Texas where 
the forest trees are now advancing over the grass plains. The 
axe and forest fires have been instrumental in changing thousands 
of acres of forest area to grass land, or in some cases to waste 
land. 




Fig. 277. Forest growth advancing on the praine, Tarkmgton Prairie, Liberty 
County, Texas. (W. L. Bray.) Where the prairies are bordered by forests, 
the tendency is in some places for the forests to advance on the prairie. 



338. Air and Light.— The chief supply of raw ma- 
terials for the plants is derived from the air and consists 
largely of carbonic acid, which in the cells of the green 
plant is decomposed, the carbon with some oxygen and 
hydrogen forming compounds which make up the plant 
tissue, while some of the oxygen is set free. The nitrogen 
of the plant comes from the soil but the soil probably ob- 
tained it originally from the air. 




Fig. 278. Sequoi. (Jeol. Survey.) The big trees, small 

trees, and shruDs {jrow in (nm-rein light zones. The big trees shade the 
smaller ones, and both shade the shrubs and ground plants. 



GEOGRAPHY OF LIFE 413 

All green plants require light. Green cells are food 
factories where water, materials from the soil, carbon 
dioxide and other gases of the air are combined to produce 
food for animals or material for other plants. The sun- 
light and the green material (called chlorophyll) of the 
plant cell seem to be the most important factors in this lit- 
tle organic laboratory where the inorganic air and mineral 
matter are changed to the organic vegetable compounds. 
The plant tissue which is eaten by animals undergoes 
further changes in the chemical laboratory of the animal 
which devours it, and it is there transformed to animal 
tissue. Some plants known as light plants require more 
light than others which are known as shade plants. In a 
forest there are several zones or strata based on the rela- 
tive amount of light. The tall trees form the upper zone 
and receive the most light, below this is a stratum of 
shrubs, then herbs, and next to the ground the green mosses 
and lichens. (See fig. 278.) 

339. Temperature.— The extremes of temperature be- 
tween which nearly all plants grow are 32 and 122 degrees 
F. Some forms of algae live in the Hot Springs of Yel- 
lowstone Park at a temperature as high as 199 degrees. 
By a special adaptation to change of conditions, plants 
lying dormant pass through cold winter seasons having a 
temperature much below 32 degrees. The distribution of 
the temperature throughout the year, and the relation of 
the temperature to moisture are important factors to con- 
sider. 

A general subdivision of the land area into plant zones based 
on temperature is as follows: 

1. Boreal, polar or cold zone, with mean annual temperature 
below 30 degrees F., contains lichens, mosses, gentians, willows, 
etc. It includes the greater part of North America north of the 
United States and high mountain areas in the United States. 



414 






PHYSICAL GEOGRAPHY 



? ^ i i 






K^:'.i»l^^ 



r^i= 







' ^^^^'""^'^^'^''"''^''''^^^ 




GEOGRAPHY OF LIFE 415 

2. Transition, or cold temperate, mean annual temperature 
30 to 40 degrees F., contains evergreens, such as spruce, fir, hem- 
lock, pine, etc. This zone forms a fairly wide belt along the 
northern United States, southern Canada, and along the Alle- 
ghany, Rocky, and Sierra Nevada mountain ranges, between the 
boreal on the mountain top and the austral of the bordering 
plains. 

3. Upper Austral, or warm temperate, mean annual tempera- 
ture 40 to 60 degrees F., contains 'deciduous trees such as oak, 
maple, beech, chestnut, etc. It covers the central United States, 
including the greater part of the plains and prairies and portions 
of the Alleghany plateau. It may be divided into the arid plains 
region of the west and the humid areas of the middle and east. 

4. Lower Austral, or sub-tropical, mean annual temperature 
60 to 72 degrees F., contains broad leaved evergreens, magnolia, 
holly, cactus, pines, palmettos, and cypress. It includes the At- 
lantic coastal plain south of the Potomac, the Gulf plains, lower 
Mississippi Valley, Texas east of the staked plains, part of 
Arizona, and the low areas of southern and central California. 

5. Tropical, mean annual temperature 72 to 82 degrees F., 
has a luxuriant vegetation, containing great numbers of climb- 
ing and air plants. It occurs in southern Florida and Cuba. 

Local Zonation. Besides the broad planetary zones of tem- 
perature just described, there are many local zones in each, some 
based on temperature, some on moisture, some on dependence 
upon other plants, some on light, and others on the kind of soil 
or rock. 

Most all large hills, and mountains are belted with different 
kinds of plants or trees, the belts being very irregular in many 
places, as the zones are determined in part by the temperature 
due to elevation, in part by light and wind, in part by the kind 
of soil. 

Most all swamps and some lakes have concentric zones based 
upon the depth of water. (See figs. 85, 90 and 92.) 

340. Control of Plant Distribution by Methods of 
Migration. — On areas in which the temperature and 
humidity favor vegetation, the plants must be distributed 
over the area in some way. In the first uplift of a lake 
bed, a coastal plain, or an island area there may be no veg- 



416 PHYSICAL GEOGRAPHY 

elation. Seeds of land plants may spread over it in va- 
rious ways as follows: 

1. Some are carried by the wind. Some seeds like 
those of the thistle and dandelion have feathery floats 
which serve as little balloons to buoy up the seed so that it 
may be carried long distances before it falls to the earth 
to sprout and start a new center of distribution. 

2. Some seeds have spines or sharp prongs by which 
they are attached to the fur or hair of animals and thus 
carried to distant points. Such are the burdocks, and 
Spanish needles. 

3. Edible seeds or seeds in edible fruit are often car- 
ried by birds or other animals to distant points, and there 
grow and multiply. 

4. Some plants have explosive seed pods that fly open 
with force and throw the seeds some distance away. A 
repetition of this process from year to year carries the 
plants over wide areas. Study the common witch hazel. 

5. Seeds and sometimes plants are carried long dis^ 
tances by rivers and ocean currents. Many oceanic islands 
obtain their plants in this way. The seeds of the cocoa 
palm are thought by some to be widely distributed by the 
ocean currents among the coral islands of the tropical Pa- 
cific Ocean. 

6. Man is one of the most important agents in dis- 
tributing plants. He transplants them to distant parts of 
the world, over mountains and across oceans or deserts. 
The food and flowering plants are carried to all lands for 
cultivation, and the seeds of weeds and other undesirable 
plants are unavoidably carried with them. The railroads 
and automobiles are important agents in this distribution. 
Many plants migrate or spread by sending out shoots, run- 
ners or underground stems. The strawberry is a good 
example. 



GEOGRAPHY OF LIFE 



417 



Certain plants at times show a very peculiar distribution. 
The Hart's-tongue fern (Fig. 280) occurs in Onondaga county, 
N. Y., in a few places only on the Onondaga limestone. It is not 
known to grow on any other rock in the county; nor is it known 
to occur in any other place in eastern United States except one 
place in Tennessee. It occurs in Ontario and is common in 
North Africa and Southern Europe. 

341. Barriers.— It seems probable that the different 




Fig. 280. Hart's Tongue fern {Scolopendrium officinarum) occurs in Onon- 
daga County, N. Y., only on Onondaga limestone. Tennessee is the only 
other locality where it is known to occur in the United States. (J. E. 
Kirkwood.) 
27 



418 PHYSICAL GEOGRAPHY 

species of plants and animals had each a starting point on 
one of the continents from which it spread over the sur- 
rounding region until it met some barrier which checked 
or stopped its further advance. Owing to the varied habits 
of different species, what would prove a barrier to one 
form might be no barrier to another. 

The question of barriers which check the spread of 
animals or plants from the center of origin is one of great 
interest but involved in too many complexities for thorough 
discussion here. 

Animals or plants that live on the bottom of the shallow 
portions of the sea on the continental shelf, find a barrier 
in the land on one side and the deep ocean on the other. 
Between these two barriers the species range along the 
shore until stopped by an obstruction of some other kind, 
the most common one being a change in temperature. Some 
forms can live only in cold water, others only in warm 
water, and others only in temperate water. Thus the reef- 
building coral polyp which cannot stan'^ a temperature be- 
low 68 degrees F. is limited by the temperature to tropical 
regions. 

To most of the plants and animals that live in the sea, 
fresh water is a barrier, and they do not enter the river 
however deep and wide the channel may be. The opposite 
is true for many fresh-water forms ; that is, the salt water 
at the mouth of a river is an effective barrier that prevents 
the spread into other rivers. There are a few forms, how- 
ever, that live in either fresh or salt water. To such forms 
the shore line is not a barrier, and they may pass along the 
shore from river to river until they reach a temperature 
barrier. 

The shore line of a large body of water is a barrier to 
most animals and plants that live on the land as well 
as to those that live in the water. Many of the land ani- 



GEOGRAPHY OF LIFE 419 

mals can cross a river or small lake by swimming but they 
find the ocean an effective barrier. 

Mountains, especially high mountain ranges, are effective bar- 
riers to most of the forms of land life both plant and animal. 
For that reason the indigenous life on the two sides of such a 
mountain range as the Rockies or the Alps is likely to be quite 
different. To most forms of plants of the austral and transition 
zones the boreal area of a high mountain range is an insurmount- 
able barrier. Seeds with balloon attachments like the thistle 
may sometimes be carried over by the wind, and burs and other 
seeds with hooks may be carried across in the fur of animals, 
but most of the plants at the base of the mountains have no 
natural means ot getting over the crest. 

Deserts, especially large deserts such as the Sahara or those 
of Central Asia, are effective barriers to most forms of life. The 
aridity of the desert is as destructive to life as the cold tem- 
perature of the mountain tops. Man, by provident forethought, 
may carry sufficient supplies of food and water to enable him to 
cross to supplies on the other side, but not so the plant or the 
wayward animal, which perishes for want of water. 

Plains are barriers to certain forms of life that flourish only 
on the mountains. The broad stretches of prairies, the treeless 
plains of the Mississippi Valley, prove a more effective barrier 
than the great rivers to denizens of the forests and the hills. 

Life of one kind may prove a very effective barrier to other 
forms of life. 

A forest is a barrier to certain kinds of plants and animals, 
while its shade is necessary for the life of others. So a thicket 
is a barrier to some forms and a protection to others. To some 
the meadow or the grassy prairie is a decided check. 

Some forms of life are dependent on others and cannot 
flourish without them. To such dependent forms the absence of 
the godfather on which they depend is a serious negative barrier 
to their advance. 

Certain insects are necessary for the fertilization of certain 
plants, and destruction of either the plants or the insects would 
cause a destruction of the other. 

Some forms are enemies to others and where they occupy 
an area they prove a decided barrier to the advance or spread 
of the new form in that direction. 



420 PHYSICAL GEOGRAPHY 

342. Effect of Vegetation on Physiography.— Some 
of the effects of vegetation on shore lines have been described 
in Chapter VI. The growth of the mangrove, the eel grass, 
marsh grass and other water plants frequently produces 
very marked changes on the position of the shore line and 
in the case of small lakes, their results are not limited to 
the shore but they fill the entire lake or marsh and make 
a fertile, dry land area of it. 

The drifting vegetation frequently lodges and forms an 
obstruction on the course of a stream where later, sand and 
gravel are deposited, sometimes turning the river from its 
channel. 

343. Indigenous and Existent Vegetation.— One 
should keep in mind constantly the distinction between ex- 
istent vegetation and the indigenous. The latter refers to 
the plants which are native to the area through geographic 
influences independent of man. The existent vegetation 
is often in large measure the result of man's influence. 

The potato, the maize or Indian corn, and tobacco are 
indigenous to America but are now widely distributed over 
the world. The peach, fig, grape, and orange are indigen- 
ous to the European continent but are now distributed in 
all countries. 

Not only are fruits, grains and vegetables widely dis- 
tributed by man, but many weeds and flowers as well. The 
thistle, for instance, has become so abundant in many local- 
ities as to cause laws to be made prohibiting any one from 
permitting it to grow on his land. 

The beautiful scarlet geranium, cultivated in our gar- 
dens and greenhouses, is indigenous to South Africa, but it 
has been transplanted by man to all parts of the world. 
It is now growing wild in great luxuriance in California, 
Australia and elsewhere. 

The following is a list of plants with the name of the country 



GEOGRAPHY OF LIFE 421 

in which they are indigenous. From this make a list of such as 
you know to be growing where you live, stating in each case the 
way or ways in which you think, it may have been transplanted 
from its original habitat. 

Fruits, etc. Apple — Europe. Apricot — America or China. 
Banana — S. Asia. Black Currant — Europe. Cherry — Asia Minor. 
Gooseberry — England. Mango — S. Asia, Malay Peninsula. Oranges 
and Lemons — Cochin China, Indo-China. Pear — Australia, all over 
Europe. Plum — common, Mt. Elbons, N. Persia. Pineapple — 
Brazil, Mexico, Guiana. Pomegranate — Persia, Afghanistan, Bel- 
uchistan. Quince — N. Persia. Raspberry — Temperate Europe 
and Asia. Red Currant — England and Normandy. Strawberry — 
Europe, Asia, N. America. Tomato — Peru. Half a century or 
more ago they v/ere considered poisonous and raised for orna- 
ment. Used to frighten the slaves before the war. Fig — Mediter- 
ranean Basin. Grape — Cultivated first probably in Asia, wild in 
N. America, Europe and Asia. Date Palm — Narrow zone from 
Euphrates to Canaries. Peanut — Brazil. Cloves — Moluccas. Red 
Pepper — America. 

Vi7ie fruits. Cucumber — ^West Indies. Gourd — Coast of Mal- 
abar and Abyssinia. Musk Melon — Asia, Mexico, or California. 
Pumpkin — Mexico or Texas. Water Melon — Egypt. 

Nuts — fruits. Cocoa-nut palm — America. Chestnut — Temper- 
ate America, Europe, Japan. Hickory — 10 species all in E. North 
America, Canada, Mexico, Walnut — North America, temperate 
Asia, S. E. Europe. Occurs fossil in Tertiary and Quaternary in 
North America. 

Miscellaneous fruits. Cactus, Prickly Pear — Native in Mexico, 
used for fruit and fodder, spineless variety now being cultivated 
in arid areas in United States. Hops — Eurasia. Lima Bean — 
Peru. Introduced in United States about 1820. Poppy — Shores 
of Mediterranean. 

Roots and tubers. Jerusalem Artichoke — Canada and United 
States. Beet — Canary Islands, Mediteranean Basin. Carrot 
— Europe and West Temperate Asia. Potato — United States 
of Columbia and Peru. Radish — W. Temperate Asia. Sal- 
sify — Borders of Mediterranean. Sweet Potato — America, some 
say Asia. Turnip — Europe, Siberia. 

Grains and seeds. Barley — Eurasia. Buckwheat — Manchuria, 
near Lake Baikal. Maize (Indian corn) — America. Oats — Europe 



422 PHYSICAL GEOGRAPHY 

and Tartary. Rice — India or China. It was used in China 2,800 
B. C. Rye — Between Austrian Alps and Caspian Sea. Wheat — 
Uncertain, very ancient, probably Euphrates Valley. Antedates 
historic records. Known in China 2,700 B. C. Found in Pyra- 
mids of Egypt of date thought to be 2,700 B. C. Durum Wheat 
— ^Long used in Russia and Southern Europe. Introduced in 
U. S. and Canada about 1900. Much more productive on the dry 
lands of the wheat belt than common wheat. Coffee — Abyssinia. 

Fibers. Cotton — South Asia. Flax — Borders Mediterranean, 
Caspian and Black seas. Hemp — Central Asia, Siberia. 

Plants used for stems or leaves. Artichoke (true) — Borders of 
Mediterranean. Asparagus — Europe, England, W. Temperate 
Asia. Alfalfa — Brought to the United States from Chile. Prob- 
ably from Asia Minor or Arabia. Celery — Europe and Asia. 
Cabbage — Europe. Clover, Purple — Asia, Aryan Nations; crim- 
son — around Pyrenees. Lettuce — S. Europe, Canary Islands, 
Algeria, E. Asia. Millet — Egypt, Arabia. Parsley — S. Europe, 
Algeria, Lebanon. Rhubarb — Central Asia. Saffron — Asia Minor. 
Sorghum — Tropical Africa. Spinach — Empire of Medes and 
Persians. Sugar Cane — S. Asia. Tea — ^Indo-China. Tobacco — 
America, perhaps Mexico, Bolivia, Venezuela. Onion — Persia, 
Afghanistan. Garlic — Europe. 

344. Uncertainty of Origin.— The original habitat of 
some of the plants is uncertain, owing to lack of definite 
precision in the ancient records. Most of the wild plants 
and many of the cultivated ones antedate definite historic 
records. Early man, ages before even the primitive stages 
of civilization, was instrumental in distributing many of 
the plants. It should be noted that many of the cultivated 
plants are varieties produced by cultivation, and new 
varieties are constantly appearing. 

The attempt to trace out the original habitat of the different 
plants has been by means of (1) Geographical Botany which deals 
with the distribution of plants, supplemented by the aid of (2) 
Archeology and Paleontology, (3) History, (4) Philology. Con- 
clusions are reached only after carefully sifting the data from 
all the above sources. 

The table given above was compiled mainly from the follow- 
ing sources: (1) Origin of Cultivated Plants by de Candolle, (2) 



GEOGRAPHY OF LIFE 



423 



Our Plant Immigrants by David Fairchild. (National Geog. Mag., 
April, 1906.) (3) Cyclopedia of American Horticulture by L. S. 
Bailey. 

With the aid of the United States Census report on agricul- 
tural products, the student should now plot on a map of the 
United States the corn belt, wheat, cotton, sugar, and rice belts, 
and endeavor with the aid of the teacher to give geographic rea- 
sons for their location. It is not a coincidence that corn is the 
chief product in one locality, rice in another, tobacco in another, 
etc. 

FORESTS 

One of the most important topics now before the Amer- 
ican people is that of the present, past and probable future 
conditions of the forests. When this country was first 
settled by white men, a large part of it was covered by 
dense forests. The early settlers chopped down the trees 




Tig. 281. Log jam at Glens Falls, N. f. This is one of the ways in which 
our forests are disappearing. (Natl. Geog. Mag.) 



424 



PHYSICAL GEOGRAPHY 



and burned them in order to clear the land for their farms. 
Later the lumbermen cut the trees by millions to furnish 
the lumber to build the cities, villages, factories, etc. So 
rapidly has cutting of the forests been carried on that ex- 
tensive areas are now bare and barren wastes, for the 
destruction begun by the chopper has been completed by 
the fires. (Fig. 282.) 

345. Effects of Forest Destruction.— On most of the 
plateau, plain, and valley areas, after the cutting of the 
forest the land was brought under cultivation, and is now 




io. -:-. ;.urth Sugar Loaf Mountain, N. H. 
a barreu waste. 



Once heavfly timbered, now 



covered with prosperous farms, but in the rocky portions 
of the mountainous and hill country, the soil is so thin or 
so poor that it cannot be cultivated, and the result is that 
unproductive and unsightly barren waste areas now mark 
the sites of former stately forests. 



GEOGRAPHY OF LIFE 



425 



In the virgin forests there was an accumulation of de- 
caying vegetation that furnished a rich soil for the forest 
trees. The fires following the cutting of the trees burned 
up the vegetable mould leaving bare rocks in place of the 
former deep, rich carpet of moss and shrubs. 

The vegetable carpet of the forest acts like a great 
sponge which absorbs and holds the rainfall, which serves 
to keep the area moist in the rainless season. The destruc- 
tion of the vegetable sponge causes most of the rainfall to 




Fig. 283. Vegetation on the surface of a forest acts like a great sponge, pre- 
venting the rapid run-off of the rainfall. Yellow pine forest in the Sierra 
Nevada Mountains. 

run over the rock surface, and thus wash into the streams 
and carry away the residue left by the fire and drought. 
The absence of the forest permits more of the rainfall 



426 



PHYSICAL GEOGRAPHY 



to run directly into the streams producing great floods, 
which means destruction to property in the valleys and 
great decrease in farm products from drought in the dry 
seasons. (See fig. 284.) 



M 


Bj^jfg^' 




f^ 


1 


B 


m 


1 










1 








% 


1 


i^^H 




r ^ M. 


^fe^i 


hi 






mm ' 


" 7**' 


R' 


m M f 




K" 




V 

A. 




*'<!^^S^H 



Fig. 284. The destruction of the forest permits the rapid erosion of the soil 
by heavy rains. Near Marion, N. 0. (U. S. Geol. Survey.) 



GEOGRAPHY OF LIFE 



427 



The preservation and protection of forest areas on the 
hills, mountains, and plateaus are of vital importance to 
the prosperity of the farms in the valleys as well as the 
farms on the upland. 

The chief products obtained from the great forests are 
lumber, wood pulp, bark for tanning, pitch, tar, and tur- 




FiG. 285. Forest preserves (in black) in the Western United States in 
1902. Some have been added since that time. The shaded areas are 
Indian reservations. (U. S. Geol. Survey.) 



pentine. The smaller trees furnish telegraph and tele- 
phone poles, ties for railways, pulp for paper and wood 
for charcoal. All of these materials are necessary in our 
great commercial industries, but the method of obtaining 
them has been wasteful and extravagant in the extreme. 



428 



PHYSICAL GEOGRAPHY 



Now, when it is almost too late to remedy it, we are begin- 
ning to realize that these products could have been obtained 
without the enormous waste. 

State and National Legislatures have at last been aroused to 
the importance of preserving or conserving the remnants of our 
former great forests for the welfare, not only of the future gen- 





^^1^^^ 




W ^^^^L ,^^B^B^^^^^BBggjjJ^WBiiKL 


M 


^^F^^^Vfl^^^HM^r]|% 




^^^fep^jfc%!o 




^H^^^ ^^'wT #• 




^7 ^T/^^^^£4^1r^ . f 






'X> / '^^^!k 




\^iLj 









Fig. 286. Forest areas (in black) on the Western United States in 1902. 
Part of the area has since been deforested by lumbermen and fires. 
Shaded areas contain a scanty growth of small trees. (U. S. Oeol. 
Survey. ) 



GEOGRAPHY OF LIFE 



429 



erations, but of the present as well. We now have considerable 
areas of forest land owned and controlled by the nation and the 
different states, from which the lumber and other products of 
the forest will be obtained, without the destruction of the forest. 
The accompanying map shows the location of the present forest 
preserves, and it is to be hoped that these preservations will 
increase in size and number from year to year. Schools of For- 
estry have already been established for the training of men to 
properly care for these forests. (Fig. 288.) 




Fig. 287. Sequoia Gigantea, Mariposa Grove of Big Trees, Cal. 
Pig. 278.) 



(See also 



The principal districts of the United States from which enor- 
mous quantities of lumber have been obtained, and which still 
contain forest remnants of great value are — 

I. New England, including Maine, New Hampshire, and 
Vermont, with their great forests of pine, hemlock, 
spruce, and cedar, and smaller forests of oak, maple, 
birch, elm, etc. 

II. The Adirondacks, with its pine and spruce. 

III. The Great Lake region, especially northern Michigan 
and Wisconsin, rich in pine and hemlock. 



430 



PHYSICAL GEOGKAPHY 




Fig. 288. I'orest rangers at work m the Texas pine forests. (W. L. Bray.) 

IV. The Alleghany Mountains and plateau with white pine, 
hemlock, hardwood in the north, and yellow pine and 
hardwood in the south. 

V. The Gulf region with yellow pine, cedar, cypress and 
oak. 

VI. The Rocky Mountain region with bull pine and spruce. 

VII. The Pacific region, the richest of all at present, with its 
great forests of redwood, pine, hemlock, spruce, and 
cedar. (Figs. 278, 286 and 287.) 

ANIMAL GEOGRAPHY 



346. Zoological Provinces and Faunas.— In general, 
animals have a greater freedom of movement than plants. 
The fact that they have the power of shifting quickly from 



GEOGRAPHY OF LIFE 431 

place to place makes them less dependent on their surround- 
ings. Most of the forms of animal life can by their power 
of movement escape many dangers, such as frost, fire and 
floods, that destroy plant life. 

A rabbit may nip off the plant, which has no way of 
escape or redress^ but when the wolf attempts to eat the 
rabbit, the laUei may escape by flight, and the former may 
move to another locality and seek other food. However, 
there are more or less well defined boundaries beyond which 
neither the rabbit nor the wolf is likely to go. The area 
over which an aggregate of associated animals wander and 
struggle for existence is called a zoological province. All 
the varieties of animals which characterize a zoological 
province constitute its fauna. 

The boundaries limiting these provinces are not always 
sharply drawn. There is usually a mingling of the faunas 
of adjoining provinces except where separated by some 
natural feature producing an abrupt change of environ- 
ment. This obstacle to the spread of life is called a har- 
rier. 

The boundaries or barriers that restrain the animals to 
certain provinces or areas are similar in many ways to those 
which control the spread of plants. All barriers are rela- 
tive and not insurmountable. Mountains, deserts, the 
ocean, changes of temperature, the relative abundance of 
other life in the form of food, shelter, or enemies are all 
barriers of much importance in the study of the distribu- 
tion of animals as well as plants. 

As with the plants, so with the animals; the ocean forms one 
of the most important of all barriers to land forms and is the 
chief one in the following broad classification of the surface of 
the earth into zoological regions: 

1. North America — including North America as far south as 
the Isthmus of Tehuantepec. Its fauna is very similar to that 
of the Eurasian region, and they have more species in common 



432 



PHYSICAL GEOGRAPHY 




GEOGRAPHY OF LIFE 433 

than any of the other provinces. Among the forms peculiar to 
it are the American bison, the musk ox, the Rocky Mountain 
goat. The monkeys, horses, and swine do not exist here as in- 
digenous forms. 

2. Eurasian, including Europe, Africa, as far south as the 
Sahara, and Asia north of the Himalayas. Here are large num- 
bers of carnivorous animals, such as the wolf and wildcat, to- 
gether with the reindeer, camel, and many varieties of wild sheep 
and goats. The monkey tribe is entirely absent. 

3. South American, including South America, the West Indies, 
Central America, and southern Mexico. The characteristic forms 
are the tapir, ant-eater, sloth, llama, monkey, and the condor and 
rhea among the birds. Equally characteristic is the absence of 
such representative families as oxen, horses, elephants, anthro- 
poid apes, and moles. 

4. African, including Africa south of the Sahara, southern 
Arabia, and Madagascar. It has the greatest number of species 
of all the provinces, and here the ungulates (hoofed mammals) 
reach their greatest development, over 150 species of this group 
being known. Well-known African animals are the giraffe, hip- 
popotamus, gorilla, aebra, ostrich, and lion. The two latter are 
characteristic but not really endemic, that is, limited to this 
province. The most notable absences are the bear and deer. 

5. Oriental, southern Asia and the islands of the East Indies 
east to the Australian region. The province has many types con- 
necting it to both the African and Eurasian regions. Species 
peculiar to it are the tiger (also in Eurasian), orang-outang, 
jungle bear, tapir and several species of antelopes. 

6. Australian, including Australia, New Zealand (sometimes 
made a sub province). New Guinea, and other smaller islands. 
It is characterized by the extreme abundance of marsupials, 
typified by the kangaroo, animals which carry the young in a 
pouch, the very peculiar duckbill, and the almost complete ab- 
sence of all five higher orders of terrestrial mammals from the 
apes to the ant-eaters. The emu, cassowary, and lyre-birds char- 
acterize the bird life. 

347. Fresh Water Life.— While some animals are able 
to exist in both fresh and salt water, yet the faunas are so 
distinct as to warrant the separate consideration of fresh- 
water forms. 



434 PHYSICAL GEOGRAPHY 

The fresh water animals, especially those inhabiting the 
larger lakes are divided into faunas, much as in the ocean. 
The forms of the surface, the shores and the bottom are 
usually here quite distinct. In the rivers the distinction 
is less sharply marked, while in the ponds and brooks it 
cannot be drawn. 

These fauna include amphibians, larval and adult, many 
varieties of fish, the larvae of many types of insects, and 
some in the adult stages, many Crustacea, many worms, 
one variety of fresh-water sponge, and other lower forms 
of life. There is great variation in these species, for the 
conditions of a fresh water existence vary greatly from 
the turbulent brook to the majestic lake. 

The. transition belt at the mouths of rivers emptying 
into the ocean, where the water is brackish, is hostile to 
most forms of life, both of salt and fresh water, and has a 
fauna peculiar to itself. 

348. Oceanic Life.— The animal life of the ocean is 
wonderfully varied and, to the interested observer, full of 
beauty. As exposed on the shores between tides, brought 
to the surface by the dredge and seine, or revealed by the 
water telescope, the myriad forms entrance the beholder 
with their profusion and tempt him to farther study. 

The distribution of oceanic faunas, while governed by 
the general principles previously outlined, is dependent 
upon special conditions which must be noticed. 

The temperature of the water is the most important 
factor in the distribution of marine animals. It is de- 
pendent upon three conditions, latitude, ocean currents 
and depth. 

In general the temperature of the surface oceanic waters Is 
higher at the equator and progressively diminishes toward the 
poles. This is, however, modified by the ocean currents which 
carry immense volumes of warm water into regions where the 



GEOGRAPHY OF LIFE 435 

waters bordering the current are considerably colder. The 
abrupt change thus produced is one of the important barriers 
existing in the ocean. Species may be able to extend their range 
widely where the change in temperature is gradual, but these 
same species often cannot endure the abrupt change of passing 
from warm to cool water or the contrary. This is especially true 
of the eggs of many of the molluscs. Hence, ocean currents often 
form important boundaries to oceanic provinces. Similarly depth 
is an important condition, for it directly affects temperature, as 
the farther from the surface the cooler the water. It is closely 
related to the effect of altitude upon the land, a few thousand 
feet vertically causing greater changes in the faunas than many 
hundreds of miles of latitude. So important is its effect that it 
is used as the basis upon which oceanic life is classified, and the 
following four great life zones are marked off by it. 

Littoral Life, that of the shore, is the best known to 
the ordinary observer of any of the oceanic zones. On 
the beach at low tide among the seaweed are found many 
interesting forms, the shore birds, many varieties of mol- 
luscs with .their oddly shaped and often brightly colored 
shells, sand worms, starfish, sea urchins, in fact, represen- 
tatives of almost every known class of animals. It is 
small wonder that men, from the ancient Greeks to the 
modern scientist, have strongly believed that where land 
and sea meet, life originated. 

The conditions of life here are greatly varied. With 
the ebb and flow of the tide, the dashing of the waves, the 
winds, the sunlight, what more invigorating environment 
can be conceived? 

Intermediate Life, (sometimes included in Littoral) in- 
cludes the life at moderate depths, ranging from low 
water level to a depth approximating 500 fathoms. It is 
not sharply separated from the Littoral and is sometimes 
included in it. It is the zone of seaweeds and corals, and 
here marine life reaches its maximum both in number and 
variety of forms. Here flourish molluscs of many types, 



436 



PHYSICAL GEOGRAPHY 



including the common clam and oyster, corals, sea anem- 
ones, sea cucumbers, crinoids or sea lilies, sponges and 




Fia. 290. Skate, one of the odd but rather abundant forms of fish in the 
intermediate zone of marine life. Cape Flattery Bank, Washington. 
(U. S. Fish Commission.) 



GEOGRAPHY OF LIFE 



437 



many lower forms, along with lobsters, crabs, and the vast 
multitude of fishes. Here life conditions, while more uni- 
form than those of the Littoral zone, have not reached the 
unvarying monotony of the great deeps. Food is abun- 
dant, the depth is not too great for sunlight to penetrate, 
and the waters are comparatively warm and undisturbed. 

Abyssal life. The boundary between the preceding 
zone and the abyssal, or deep sea, is not sharply defined, 
many species passing far to each side of the arbitrary 




Pig. 291. One of the deep sea fishes. They live in perpetual darkness in the 
cold waters on the floor of the ocean basins. Taken from a depth of 
several thousand feet. (Smith. Inst.) 



438 



PHYSICAL GEOGRAPHY 



depth taken as the dividing line. Conditions of life here 
are extremely uniform, no light, a uniform temperature 
approximating 34 degrees, great pressure, little motion of 
water, and a high percentage of oxygen. Few plants live 
here, and the animals are carnivorous, feeding upon each 
other and animal remains, which slowly sink down from 
the surface. 




Fig. 292. One of the odd-shaped forms of life from the bottom of the deep 
sea. (Smith. Inst.) 



Abyssal animals are much the same the world over, and in 
spite of the seeming adverse conditions under which they live, 
the life is quite varied, including all the main types from the 
fishes down. Many of the forms are extremely odd and curious. 
Knowledge of deep-sea life has been greatly extended during the 
recent years by the use of the dredge. (Figs. 291 and 292.) 

Pelagic life includes those forms which habitually live 
on the surface of the open sea or at moderate depths below 
it. Here under the favorable condition of abundant sun- 
light and moisture, and with many minute forms which 
furnish food to the larger animals, a rich and varied life 
is found. Whales, many forms of fish, pteropods and 



GEOGRAPHY OF LIFE 439 

cephalopods among molluscs, crustaceans, and vast num- 
bers of lower forms such as the Portuguese man-of-war, 
jelly-fish, infusoria and radiolaria are all pelagic in their 
habits. 

Between the surface zone with its rich fauna and the ocean 
bottom with its scanty fauna is the great body of oceanic waters 
which, so far as our present knowledge shows, is almost devoid 
of life. 

THE GEOGRAPHY OF MAN 

349. Distribution of Mankind.— At the World's Fair 
in St. Louis one could see men from all the continents and 
many of the larger islands of the world. There was a 
great diversity in color, size, and other physical features. 
Are these people all from one parent family, and if so, 
where was the home of that family, and how came this 
great diversity in racial characteristics? 

The original habitat of man is thought to be in western 
Asia, from whence descendants migrated in different direc- 
tions. Probably the principal factor in the variations in 
color, size, facial features, and mental development was 
geographical environment. After a long period of time 
the slow changes finally resulted in a great many races 
and tribes which are sometimes grouped in the following 
four races : 

1. The Ethiopian or black race Number— 173,000,000 

2. The Mongolian or yellow race *' 540,000,000 

3. The American or red race ** 22,170,000 

4. The Caucasian or white race '* 770,000,000 



Total 1,505,170,000 

The original habitat of the Ethiopian race was Africa 
south of the Sahara, Madagascar, and many of the East 



440 PHYSICAL GEOGRAPHY 

Indies. The second probably started from the Tibetan 
table-land. The third occupied the new world, America. 
The fourth race probably started in North Africa. From 
the original habitat these races have spread over the world 
and are now mingled on all the continents. 

For many centuries, the Eed man held undisputed 
sway in what is now the United States. About four cen- 
turies ago the white man first came in small numbers, then 
in larger numbers, and later he brought the Black man 
first as a slave. The yellow man found his way across the 
Pacific Ocean and entered our western border. We now 
have all four races in large numbers. So on all the con- 
tinents there is a commingling of different races. 

It must be remembered that the above classification is sn 
arbitrary one, and only one of many attempts to classify the 
human family. Each of the divisions contains many classes and 
tribes, each with its own characteristics of physical and mental 
traits. There are distinctions in size and shape of the skull, 
color and texture of the hair, language, and above all, mental 
development. A much more elaborate classification is given in 
the Standard Dictionary under Man. 

The Mongolians have probably an older fairly authentic his- 
tory than any of the others, but as far back as their history ex- 
tends, the Chinese and the Japanese branches are separate. 
They have retained their present national characteristics through 
a longer period of time with less changes than any of the other 
nations. 

The peoples that have passed through the greatest changes 
and most rapid development in civilization are some of the 
branches of the Caucasian race. In none of the nations, how- 
ever, does authentic history extend back to the beginning of the 
race. That the different races are branches of one common fam- 
ily is an inference or deduction based on a study of the whole 
human family in their physical and mental characteristics, and 
relations to each other and the other animal forms. The origin 
of the human family is involved in obscurity. 

350. Influence of Geography on Man.— While man by 



GEOGRAPHY OF LIFE 441 

his ingenuity has prevailed over the forces of Nature in 
many ways, yet it still remains true that he is greatly in- 
fluenced by his geographic surroundings. The climate, 
topography, proximity to the sea, all wield a wonderful 
influence over man. 

351. Climate.— All the great nations of the world have 
had their rise and growth in the temperate climate. This 
is not a coincidence. Man may carry civilization into cold 
and hot climates and may foster it for a time, but the fact 
remains that the development of the great civilized nations 
has been in the temperate zone. 

The continued heat of the tropics tends to make one 
languid and lacking in enterprise. The warm climate re- 
quires little clothing or shelter to protect one from the ele- 
ments. The great abundance of tropical fruit makes the 
food supply an easy problem. Thus the great incentive 
to provide food, clothing, and shelter which arouses man to 
his best eft'orts in a cooler climate is lacking in the warm 
tropics. 

In the polar regions the cold is so intense and long con- 
tinued that it is a perpetual struggle for existence ; hence, 
one does not find the opportunity to cultivate the mind 
and surround himself with the luxuries and comforts of 
modern civilization. 

In the temperate region the cold winters rouse man to 
exertion to provide f^od, clothing and shelter, and yet the 
cold is not so severe as to dwarf his energies or prevent the 
development of his mental and physical powers. 

352. Influence of Topographic Forms.— As already 
stated the surface features greatly influence the distribu- 
tion of the population, as well as the occupations of the 
people and many of their customs and habits. 

The most densely populated areas are generally on the 
plains because facilities for travel and transportation are 



442 PHYSICAL GEOGRAPHY 

there superior and favor the commercial and manufactur- 
ing industries. Such conditions also favor agriculture 
and hence abundant food supply. 

Life in the high mountains tends toward the isolation 
of groups of people and the development and perpetuation 
of local customs. There is little communication and in- 
tercourse with outside people. In the absence of mineral 




Fig. 293. Farm house in the Boston Mountains. In many mountainous 
districts farming is not a profitable industry. 

products or large forests the mountain people are liable to 
be poor and live a very simple life with few luxuries. In 
the primitive and pioneer stagas the people in the moun- 
tains depend largely for support upon hunting and fishing. 
The charm and the freedom of such a life overcomes in 
large measure any desire for so-called luxuries, and, hence, 
any incentive or opportunity for wealth. When the moun- 



GEOGRAPHY OF LIFE 443 

tain people are dependent upon agriculture, the barren 
soil and rough surface prevents any profit more than mere 
subsistence; and, hence, the hard struggle for food and 
shelter hinders, if it does not prevent advancement in cul- 
ture, learning and conveniences of civilization. (Fig. 293.) 
353. Proximity to the sea generally favors easy com- 
munication with other nations and countries, and hence 
fosters the commercial spirit which results in wealth and 
cosmopolitan ideas. In past centuries many of the sea- 
faring nations were war-faring as well and conquered by 
might where at the present day the battles are fought 
more on manufacturing and commercial lines. 

A few examples will best illustrate the influence of geo- 
graphic conditions on man. 

The Eskimo gives all his time and energy to the chase. 
He has no chance to raise vegetables, even if he desired. Hav- 
ing but a limited supply of fuel he learns to depend largely on 
the conservation of his own bodily heat for warmth, so he 
dresses in furs, eats fat and lives in ice houses. His life is not 
devoid of adventure, but there is little incentive to advancement, 
and the Eskimos to-day are probably no better off than their 
ancestors were centuries ago. 

The Pigmies of the African forest live in a rude shelter of 
bushes quickly constructed and, hence, it is no great hardship 
to leave it and migrate to a distant part of the forest. They have 
no agriculture and live on nuts and wild animals which they cap- 
ture in snares and pits. Having no reserve supply of food they 
are frequently subject to hunger and sometimes starvation. 
There is no development of the mental faculties, and they remain 
but little superior intellectually to the animals which they pursue. 

Emigrants from Europe, scarcely three centuries ago, entered 
the present area of the United States. They have cut down the 
forests, cultivated the soil, built cities and factories, extended 
steam railways and electric lines far and wide over the land. 
Many boats ply the inland waters and hundreds of vessels sail 
to and from foreign lands. Telegraph lines extend to distant 
parts of the earth. One may read in the evening papers an ac- 
count of any or all of the important events that have happened 



444 PHYSICAL GEOGRAPHY 

anywhere in the world during the day. If one desires he may 
without leaving his chair talk by telephone with any of his friends 
within a radius of several hundred miles. He has in his service 
many of the varied products of the world; fruits of the field, 
garden, mine, and factory are at his command. What a contrast 
is this life with that of the Eskimo or the African Pigmy! 

The difference in habit, customs, civilization, and develop- 
ment of these peoples is not entirely due directly to climate, but 
partially to racial differences. The North American Indian that 
was here before the European, was and is yet, in his natural state, 
as far below the white in civilization as he is superior to the 
Pigmy. How far these racial differences are due to climatic con- 
ditions in past ages is a subject worthy of consideration. 

The influence of geography on the migrations of man, in the 
founding of cities, in the construction of highways, has been sug- 
gested at different places in the preceding pages and will be con- 
stantly suggested to the student of history and geography in all 
his study and travel. 

The migrations westward from the early settlement at Phil- 
adelphia first spread out in the Chester — "the Little Valley" — 
and later in the "Great Valley" because in these valleys was a 
rich limestone soil more productive and more easily tilled than 
that on the bordering hills. The further migrations were 
through the water gaps into the other valleys of the Alleghany 
Mountains, where the people lingered long before making the 
difficult and dangerous journey up and over the rocky forested 
plateau extending west to the Ohio region. Besides the great 
diflSculty and danger in traveling, the climate of the plateau is 
more severe and the soil less fertile than in the sheltered valleys 
and coves. Hence in the valleys they lingered until the French 
were pouring into the Ohio Valley by ascending the St. Lawrence 
and crossing the Great Lakes and descending the tributaries of 
the Ohio. (See fig. 294.) 

Pittsburg was a strategic point in the early colonial days and 
as Fort Duquesne and Fort Pitt, it was the scene of bloody con- 
flicts between the nations. The early settlers knew nothing of 
the great coal beds and the deposits of oil and gas that have been 
so instrumental in making this one of the great manufacturing 
cities of the world, but they could see its great advantages as a 
commercial center, and hence, the feverish haste of the French 
to- get possession, and of the English to dispossess them. 



GEOGRAPHY OF LIFE 



445 




446 PHYSICAL GEOGRAPHY 

It is only necessary to study the geographic location and 
surroundings of New York, Boston, Chicago, Buffalo, and the 
other great cities to see that their growth has been governed by 
geographic features, which frequently were not perceived by the 
people at the time, but which governed them, nevertheless. 
The student from previous reading, study and observation should 
put in writing the geographic reasons for the location and growth 
of our great cities. These should be compared in the class-room 
and supplemented by explanations from the teacher. 

354. Influence of Man on Geography.— Man is not a 
mere passive agent. While he has been influenced in 
many ways by his geographic surroundings, he has had a 
very marked influence on them in return. As evidence of 
this one needs but to compare the United States of to-day 
with its condition four centuries ago. 

A large part of the dense forests has been destroyed. 
The freshly plowed soil exposed to the rains has been 
washed in large quantities into the streams and carried to 
or toward the sea. In the construction of cities, highways, 
and railways, hills have been cut through, sometimes cut 
down, valleys and lakes have been filled or partly filled. 
Streams have been diverted from their courses. Great 
dams or lakes have been constructed in some places and 
destroyed in others. Many of the wild animals have been 
wholly or partly destroyed and domestic animals have 
taken their place. Orchards have replaced the forest in 
part, grains and vegetables have taken the place of the 
wild plants over large areas. 

Canals have been dug across divides connecting differ- 
ent river basins. The Chicago Drainage Canal carries 
water from Lake Michigan into the Mississippi River, 
water that under natural conditions would drain through 
the St. Lawrence River. We are even attempting to con- 
nect the Atlantic and Pacific Oceans by an artificial chan- 
nel. 



GEOGRAPHY OF LIFE 447 

The steel bands of the railway connect the oceans at 
several points and a considerable portion of the intervening 
territory is covered with a lacework of steel rails over 
which millions of tons of material are being shifted from 
one part of the country to another, and, in connection with 
the steamboats, part of it even to -distant countries. 

Great stone quarries in many places have left holes in 
place of hills. Clay, sand, gravel, marl, and ore pits are 
in many places so numerous and extensive as to entirely 
change the surface features of the area. 

In many places the mountains and plateaus are bored 
and tunnelled by numerous excavations to an extent al- 
most beyond belief. Besides the large mine openings man 
has bored thousands of deep holes through which have been 
taken vast accumulations of oil, and gas. 

Elsewhere through artesian wells he has brought the 
ground water to the surface in arid areas and thus added 
to the fertility of the country. In other localities where 
there was too much water and the land was swampy, 
malarial, and unproductive, he has by surface or sub-sur- 
face draining made it dry, healthful, and productive. 

REFERENCES 

Coulter, J. M., Plant Relations, D. Appleton & Co. 

Clements, Research Methods in Ecology, University Pub. Co., 
Lincoln, Nebraska. 

Schimper, Planzen Geographie, Jena, Gustav Fischer. 

Cowles, The Physiographic Ecology of Chicago and Vicinity, 
Bot. Gaz. 31, 73, 1901. 

Wolle, Diatomaceae of N. America (Hid. with 2,300 figs.) 
The Comenius Press, Bethlehem, Pa., 1894. 

Bray, Distribution and Adaptation of the Vegetation of Texas, 
Bull. 82, University of Texas. 

Lamson, Scribner, Grasses as Sand and Soil Binders. Year- 
book, U. S. Dept. of Agr., 1894. 



448 PHYSICAL GEOGRAPHY 

Hill, Physical Geography of the Texas Region, U. S. Geol. 

Surv. Topographic Atlas. 
Hitchcock, Methods Used in Controlling and Reclaiming Sand 

Dunes. U. S. Dept. of Agr., Bur. of Plant Industry, 

Bull. No. 57. 
Lloyd & Tracy, The Insular Flora of Miss, and La., Dept. of 

Botany, Columbia University, No. 174. 
Webber, The Water Hyacinth and its Relation to Navigation. 

U. S. Dept. of Agr., Bull. No. 18. 
Allen — 1. The Geological Distribution of Animals, Bull. 

U. S. Geol. and Geog. Surv. of the Territories, .Vol. 

IV, pp. 313 to 377. 
2 The Geological Dis. of N. American Mammals, Bull. 

Am. Mus. Nat. His., Vol. V, pp. 199-243. 
Heilprin, Angelo, The Geog. and Geol. Dist. of Animals. Int. 

Sci. Ser., London and New York, 1897. 
Osborne, The Rise of the Mammalia in N. America, N. Y., 

1893. 
Merriam — 1. The Geog. Dist. of Life in N. America, Proc. 

Biol. Sur., Washington, Vol. VII, 1892. 

2. Life-Zones and Crop-Zones of the U. S., Bull. No. 

10, Dept. of Agr. Div. of Biol. Surv. 



CHAPTER XII 

PHYSIOGRAPHIC REGIONS OF THE UNITED 
STATES* 

The area of the United States is so large and diversified 
that it contains numerous examples of all the different 
physiographic types previously described. 

The entire area of the United States is conveniently 
divided for study into: (1) the Eastern or Atlantic region; 
(2) the Lake region; (3) the Central' or Mississippi 
region; (4) the Southern or Gulf region; (5) the Western 
Interior region and (6) the Pacific region. 

355. 1. The Eastern or Atlantic Region.— Under 
this heading is included the eastern part of the United 
States next to the Atlantic Ocean but not confined to the 
area that drains into it. The southern part of the moun- 
tainous area drains westward into the Mississippi, yet 
physiographically it belongs to the same province as the 
northern part which drains eastward into the Atlantic. 

A. The Atlantic coastal plain is the part bordering 
the seashore and may be divided into three portions; (a) 
the submarine plain, corresponding to the part of the con- 
tinental shelf on our eastern seaboard. It is now under 

*The different regions may be studied by following the order of geo- 
graphical position, that is, beginning at one side, as the east, and taking up 
each region in turn across the country to the west side, or by studying all 
the areas of the same feature at one time, as, for example, all the moun- 
tains first, then the plateaus and plains. While there are advantages in each 
method the author favors the first. However, any teacher preferring the 
second method can readily use it with the map and data given. In either 
case only a brief outline can be given in a general treatise of this kind, 
and the student will find it advisable to take up a few of the areas more 
thoroughly by utilizing some ' of the reference works cited. A small map 
showing all the different regions should be made by the student. 
29 449 



450 



PHYSICAL GEOGRAPHY 




Fig. 295. Photograph of Relief map of part of Eastern and Central United 
States. (E. E. Howell.) Trace out on this the physiographic regions 
mentioned in the text. 



THE PHYSIOGRAPHIC REGIONS 



451 



the sea but parts of it have been land area at times in the 
past and probably will be in the future; (b) the tidal 
flats and coastal marshes comprise the portions of the coastal 
plain that are at least partly exposed during the low tide 
and are largely covered with water during high tide. In 



*? - 



"^l^'i^ 



Fig, 296. View on the low plains of Central Florida. The alligator and 
rattlesnake are abundant where not destroyed by man. The vegetation is 
typical of the swamp areas. Pine forests where not destroyed occur on 
the more elevated portions. (A. M. Reese.) 

some places they are covered with salt water, in others 
brackish water (a mixture of salt and fresh) and in others 
by fresh water. Some of the fresh water swamp areas are 
above high tide but intimately connected with the tidal 
areas, (c) The emerged plains include portions of the 
coastal plain elevated above high tide, extending back in 
places many miles from the sea. They are now covered 



452 



PHYSICAL GEOGRAPHY 




THE PHYSIOGRAPHIC REGIONS 453 

with layers of sand, clay, and marl that were deposited 
over the former sea-bottom, and contain a great deal of 
valuable farm land. In places the tidal fiats are absent 
and the emerged plains are separated from the submarine 
plain by the shore line. What is the relation of the Pall 
Line to this area? (See sec. 116.) 

B. The Piedmont-New England Plateau, Inland from 
the sandy coastal plain and separating it from the moun- 
tains is an area covered with hard, crystalline rocks and 
having a hilly, irregular surface. (See fig. 297.) 

Since it lies at the foot of the higher mountains further 
west it is called i)ied- (foot) mont (mountain). Since it 
is elevated above the more recent coastal plain so promi- 
nently it is called a plateau. It varies in width through 
the Middle and Southern states, reaching its greatest width 
in New England. The greater part of the piedmont belt 
was in early geological times covered or partly covered 
with mountains which during long ages were worn down 
nearly to a plain, that is, to a peneplain. The entire area 
was elevated again and the streams cut many deep valleys 
into the uplifted plain, dissecting it into a complexity of 
hills and valleys. The Fall Line separates this from the 
coastal plain. (Figs. 194 and 297.) 

C. The Appalachian Mountain Area. The somewhat 
complex mountainous area in the Eastern United States 
is divided into four regions each of which in turn has 
many complexities. (Fig. 295.) 

(a) The first range of mountains bordering the Pied- 
mont plateau on the west is called the South Mountain in 
Pennsylvania and the Blue Ridge in Virginia and farther 
south. It is a very irregular range, both in size and struc- 
ture. It is composed in part of very old rocks, hard and 
crystalline, in part of brown sandstones and shales of more 
recent age. 



454 PHYSICAL GEOGRAPHY 

(b) The Great Valley of the Appalachians is the great 
depression that separates the South Mountain Blue Ridge 
from the ridges of the Alleghany Mountains on the west. 
It is a broken diversified area, quite hilly in places, and 
very different from an ordinary river valley. It is only 
when we consider it in its relation to the mountains on 
each side that it appears as a valley. Part of the area is 
underlain by limestone and the other part mostly by shale. 
In the residual clay overlying the rocks are vast quantities 
of iron ore and white clay. 

The great Shenandoah Valley of Virginia, the Cumber- 
land and Lebanon Valleys in Pennsylvania lie in and form 
part of the Great Valley that extends from Lake Cham- 
plam, New York, to Alabama. 

(c) Bordering the Great Valley on the west are the 
Alleghany ridges which have a general northeast-south- 
westerly trend. The ridges have a somewhat uniform 
height and are quite variable in length. Some extend only 
a few miles, some a hundred miles or more. In places 
they are like upturned canoes, the "Canoe Mountains," in 
other places they twist and turn in different directions. 
(See figs. 238 to 242.) 

The ridges are almost all composed of sandstone which 
on disintegration gives rise to a quite unproductive sandy 
soil. The valleys separating the ridges have generally a 
limestone soil which is quite fertile. The valleys are 
decked with valuable farms and the ridges formerly 
forest-clad are now covered only with rocks and bushes. 
Their chief products at present are pure water, pure air, 
huckleberries and rattlesnakes, of which the first and third 
can be transported to the towns in the valleys but the other 
two can be best enjoyed by a trip into the mountains. 

(d) Immediately west of the Alleghany ridges is the 
Appalachian plateau, including the Alleghany plateau on 



THE PHYSIOGRAPHIC REGIONS 



455 



the north and the Cumberland plateau on the south. It is 
bounded on the east by a steep irregularly notched escarp- 
ment facing the narrow valley that separates it from the 
Alleghany ridges, and it slopes westward gradually merg- 




FiG. 298. Typical view on the Alleghany Plateau. Potomac Valley, near 
Chaffee, Md. (Md. Geol. Survey.) The plateau is covered with forests 
(where not destroyed) and parts of it are underlaid by beds of coal, and 
pools of petroleum and natural gas. It is dissected by many rivers which 
flow in deep valleys. 

ing into the Mississippi and Lake Plains. It is deeply 
trenched by great numbers of streams so that in many 
places it resembles an irregular mountain mass. (See 
Charlestown, W. Va., topographic sheet.) There are a 
few elevated mountain ridges in this plateau area, such as 
Chestnut and Laurel Ridges in Western Pennsylvania. 

The Catskill Mountains form the northeastern end of 
the Alleghany plateau. Many of the more rugged portions 



456 



PHYSICAL GEOGRAPHY 




THE PHYSIOGRAPHIC REGIONS 457 

further south are locally known as mountains, but like the 
Catskills they are mountains of circum-erosion. The Erie 
Canal and New York Central Railway run just north of 
the plateau escarpment from Albany to Rochester. (Fig. 
299.) 

From a good map make a list of the streams that drain 
(a) into the Atlantic from this plateau, (b) into the Ohio, 
(c) into the Great Lakes. 

(e) The Adirondack Mountains lie north of the Alle- 
ghany plateau and are separated from it by the Mohawk 




Fig. 300. View of the Adirondack Mountains near Keeseville, N. Y. The 
bordering plain in the foreground. 

Valley. The rocks of the Adirondacks ally them closely to 
the New England plateau area from which they are sepa- 
rated by the Champlain Valley. They are older than the 
Alleghany, the Catskill, or the Green Mountains. (Fig. 
300.) 



458 PHYSICAL GEOGRAPHY 

356. II. The Lake Plains.— Lying west of the Adi- 
rondacks and extending west along the border of the Great 
Lakes is a strip of variable width forming the lake plains. 
The Lake Ontario plain is sharply divided on the west 
from the Erie plain by the escarpment at the mouth of the 
Niagara river gorge, but further east in New York State 
the two plains merge into one. In Michigan, Wisconsin, 
and Minnesota in places the plains are rugged and diversi- 
fied by many hills, the area in places resembling the Pied- 
mont plateau of the Atlantic region. The Lake Plains are 
sometimes divided for more detailed study into (1) the 
Superior lowland, (2) the St. Paul-Madison upland, (3) 
the Green Bay lowland, (4) the Michigan-Huron-Erie low- 
land, (5) the Lansing upland, (6) the Niagara upland, 
(7) the Ontario lowland. 

357. III. The Mississippi Valley Region.— If one in- 
cludes in the Mississippi Valley all the area between the 
Alleghany plateau and the Rocky Mountains it forms one 
of the largest and most important of the physiographic 
regions of the United States and one so diversified that it 
can be readily subdivided into a number of minor areas. 
As already stated there is no sharp line of separation be- 
tween the eastern side of the valley and the Alleghany 
plateau. 

(a) The northern part of Indiana, nearly all of Illi- 
nois, portions of Missouri, Iowa, Kansas, Nebraska, Min- 
nesota, and the Dakotas are covered with great stretches 
of treeless plains known as prairies, in some places rolling 
in others remarkably level for long distances. In the wild 
state these prairies were covered with tall grass and al- 
most devoid of trees. Since it has been brought under 
cultivation many orchards and groves have been planted. 

(b) The western part of the valley, extending from the 
prairies to the Rocky Mountains and covering portions of 



THE PHYSIOGRAPHIC REGIONS 459 

Colorado, Kansas, Nebraska, Wyoming, Montana, and the 
Dakotas is known as the Great Western Plains. A large 
part of the area has a semi-arid climate, too dry for farm- 
ing (other than grazing), without irrigation. Some por- 




FlG. 301. View on the (xreat Western Plains. Notice tlie clweumg 
house on the horizon. (U. S. Geol. Survey.) 

tions of the area are watered from artesian wells (sec. 58).' 
Other portions are irrigated from water ponded in the 
deep canyons of the Rocky Mountains. 

(c) There are several mountain areas over the western 
part of the Mississippi Valley. The Washita Mountains 
south of the Arkansas river in the states of Arkansas and 
Oklahoma are ridge mountains similar in structure and 
age to the Alleghany ridges of the Atlantic area. 

(d) The Boston Mountains lie north of the Arkansas 
river in Arkansas and Oklahoma. They consist in part of 
dissected plateaus but there has been some folding and 
faulting in places. They belong in the same class with the 
Catskill Mountains, but are somewhat more complex. 



460 



PHYSICAL GEOGRAPHY 



(e) Covering a considerable area north of the Boston 
Mountains is an upland area known as the Ozark Plateau. 
The Boston Mountains stand on and form a part of the 
plateau. (See fig. 295.) 

(f) The Black Hills in western Dakota and Eastern 
Wyoming are dome mountains in the same class with the 
Adirondacks but more recent in age and less complicated 
in structure. In the area bordering the Black Hills, and 
to a less degree elsewhere in the western plains, there is a 




Fig. 302. "Granite Needles" near Harney Peak in the Black Hills. (U. S. 
Geol. Survey.) Part of the area is very rugged. There are other forms 
of hills and ridges in the Black Hills area. 

type of topography kno^\^l as the Bad Lands. It consists 
of a very irregular surface with a maximum number of 
deep gullies and narrow ridges with sometimes fantastic 
shapes. (Figs. 303, 39, 40, 41, and 234.) 

(g) The Delta of the Mississippi is a vast stretch of 
lowland covering a large part of Louisiana. It is covered 



THE PHYSIOGRAPHIC REGIONS 



461 



with a network of streams, bayous, and lakes. The greater 
part of the lowland area is very fertile as it is composed of 
rich alluvium carried down by the Mississippi river. The 
water table is so near the surface that the region never 




Fig. 303. "Chapoau do Ferame" in the Bad Lands of S. Dakota. (V. H. 
Geol. Survey.) Columns 40 ft. high. Notice the effect of alternating 
hard and soft layers. The erosion is partly by occasional heavy rains, 
partly by winds. 

suffers from drought but sometimes it does from floods. 
It is placed in the Mississippi Valley region but it is just 
as properly a portion of the Gulf region as it has all been 
reclaimed from the Gulf. 

(h) The flood plain of the Mississippi is the lowland 
area bordering the river that is subject to periodical over- 
flow from floods in the river. It varies in width from a 
few miles to 100 miles or more. 

358. IV. The Gulf Region is in part a continuation 



THE PHYSIOGRAPHIC REGIONB 



463 



of the Atlantic coastal plain, consisting like it of a sub- 
marine plain, coastal marshes, and emerged plain. The 
latter extends north into and partly forms the Alabama- 
Georgia cuesta* in the east and the Texas cuesta in the west. 




Fig. 305. View on the staked plain or cuesta of Texas, (W. L. Bray.) It 
is an arid region with little vegetation or surface water. 

The Texas cuesta is also known as the Llano ^Estacado or 
staked plain. The staked plain is the southern extension 
of the Great Western Plains and like them is too arid for 
agriculture except when irrigated. It terminates on the 
west at the Pecos Valley. The Trans-Pecos country be- 
tween the Rio Grande and the Pecos Rivers contains the 
San Francisco or Trans-Pecos Mountains. 



*A cuesta is a low ridge with a steep descent on one side and a gentle 
slope on the other. On most cuestas the gentle slope is towards the present or 
former sea shore. 



464 



PHYSICAL GEOGRAPHY 



359. V. The Western Interior Area.— (a) Between 
the Great Plains and the Sierra Nevada is a stretch of up- 
land country composed of mountains, plateaus, plains, and 
basins. The great range of mountains bordering the plains, 
the hiofhest and most massive in the United States is com- 









^g^;yt»^.^^_ T^-^ 



Fig. 306. Outcrop of a great sandstone ledge at the end of the Freezeout 
Mountains in Central Wyoming. One of the subsidiary mountains in the 
Rocky Mountain system. 



monly known as the Rocky ^fountains. They form the 
backbone of the continent and consist of great complexities 
of mountains rather than a simple range. There is a de- 
pression or break in these mountains through central 
Wyoming followed by the Union Pacific railway, south of 
which the mountains have been called the Park Mountains 
and those north the Stony Mountains. In both areas there 



THE PHYSIOGRAPHIC REGIONS 



465 



are many portions to which more local names have been 
given, some of which are the Gallatin, Laramie, Freezeout, 
Elk, and San Juan Mountains. Pike's Peak at Manitou 
is one of the best known of the many high peaks, and is 
the highest mountain peak in the world which has a rail- 
way extending to its summit. 




FiG. 307. Western escarpment in (he J-'ict-zcout Mountains. ( U. G. Cornell). 
A typical "Hog Back" Ridge, one of the common physiographic features 
of the Rocky Mountain region. 

The Hog Back ridges forming the foothills of the 
Rocky Mountains are characteristic features of this region. 
They are narrow, sharp-crested hills formed by the out- 
cropping edges of hard layers of rock between softer layers 
which were turned up nearly vertical in the uplift which 
formed the mountains. They vary in height from 100 ft. 
to 1,000 ft. or more. (See figs. 307, 308, and 35.) 

Numerous rivers have cut wonderful canyons deep into 
30 



466 



PHYSICAL GEOGRAPHY 



the rocky center of these great mountains, exposing to 
view many valuable veins of gold, silver and other metals. 




i-'ia. 308. "Cathf'dral Spires" in in.' ...n.l.n ui tin- (io.ls. O • _^- ^-'-'l- 
Survey.) Formed by irregular erosion on tlic outcrop of the vertical beds 
of red sandstone. 



THE PHYSIOGRAPHIC REGIONS 



467 



(b) The Wahsatch and Uintah Mountains in Utah, the 
Basin Ranges in Nevada, Idaho, California and Arizona 
are other large and picturesque mountains in the interior 
region. 




Fig. 309. Knight's Butte, Central Wyoming. Copyright, 1900, by U. G. 
Cornell. A view on the dissected high plains in the midst of the 
Stony Mountains. 



(c) The great Colorado Plateau west of the Park Moun- 
tains and the Columbian Plateau west of the Stony Moun- 
tains are two of the largest and highest plateaus in Amer- 
ica. The first is deeply trenched by the great Colorado 
River and its tributaries and the second by the Columbia 
and Snake rivers. In both of these are some of the deep- 
est and most picturesque canyons in the world. 

(d) The Interior Bami lying between the two plateaus 
and the Pacific mountains really consists of a great number 
of basins containing numerous lakes, fresh, salt, and alka- 



468 



PHYSICAL GEOGRAPHY 




Fig. 310. Photograph of relief map of portion of the Colorado Plateau and 
the Grand Canyon of the Colorado. (E. E. Howell.) 



line. The entire basin region has an arid climate but ir- 
rigation has made some portions of it quite fertile and 
prosperous. Much of the area is covered with plains of 
sand, alkali, or salt. (Figs. 312, 274, 275, 79, 80, and 84.) 
Some portions of this area are the lowest on our con- 
tinent. The Salton basin in southern California is 287 
feet below sea level and Death Valley in eastern California 



THE PHYSIOGRAPHIC REGIONS 



469 



is 276 feet below. Both of these areas belong by position 
to the Pacific region but the fact that they are interior 
basins, places them physiographically in the Western In- 
terior region. They contain large deposits of salt, borax, 
soda, and other salts. A great many mountains of the 
class known as block mountains occur in the Interior Basin. 
(See fig. 312 and sec. 266.) 




i'lG. bll. View in the Grand Canyon of the Colorado, the deepest 
and one of the most picturesque canyons in the world. (A. R. 
Crook.) 

360. VI. The Pacific Area.— In its broader features 
the Pacific region consists of two mountain ranges with 
the Great Valley of the Pacific separating them. The 
eastern of the two ranges is known as the Sierra Nevada 
at the south and the Cascade Mountains farther north. 
The western range is called the Coast Mountains. The 
central portion of the Great Valley is the Valley of Cali- 
fornia, occupied in part by the Sacramento and San Joaquin 
rivers, the southern extension is through the Salton Basin 



470 



PHYSICAL GEOGRAPHY 



into the Gulf of California. Northward it is continued 
as the Sound Valley through Oregon and Washington. 
This valley on the west side of the continent corresponds 
to the Great Valley of the Appalachians on the east and 
adds to the symmetry of the national area. 




Fig. 312. One of the block mountain xidges in the Interior Basin region. 
(D. T. McDougal.) 

This great western valley is more distinctly divided 
into sections than is its eastern prototype. The Klamath 
Mountains in Oregon form a mountain mass connecting 
the Sierra Nevada, Cascade, and Coast Mountains across 
the valley. In southern California also the Sierra Nevada 
and Coast Mountains join and cut off the Salton Valley 
from the Valley of California. 



THE PHYSIOGRAPHIC REGIONS 47V 

The student should make a classified list of the physiographic 
features of the United States, locating each by states as follows: 

I. Plains. • 

1. The coastal plains. 

2. The interior plains that were at one time coastal 
plains. 

3. The cuestas. 

4. The prairies. 

5. Lake plains. 

6. Alluvial plains — deltas. 

II. Plateaus. 

1. Arid plateaus. 

2. Forested and cultivated plateaus. 

III. Mountains. 

1. Folded ridge mountains. 

2. Domed mountains. 

3. Mountains of erosion. 

4. Block mountains. 

5. Volcanic mountains. 

IV. Rivers. 

1. Which drain into the Atlantic? the Pacific? Gulf of 
Mexico? 

2. Which fiow through deep canyons? 

3. Which have high falls? 

4. Which are valuable aids in navigation? 

5. Which drain fertile farm land? 

6. Which drain coal basins? 

7. Which have large deltas? 

8. Which have none? 

9. Which have no flood plains of any size? 

V. Name and locate the salt lakes. 

On a blank map of the United States sketch in the boundaries 
of Lakes Iroquois, Passaic, Agassiz, Bonneville, Lahontan and 
any other fossil lakes known. See references in chapter on lakes. 



BEFEBENCES 

1. ;powell, Physiographic Regions of the United States^ 

2. Davis, New England Plateau. 

3. Willis and Hayes, Appalachian Mountains. 



472 PHYSICAL GEOGRAPHY 

All of the above published by the American Book Co. 

4. Russell, Rivers of North America, G. P. Putnam's Sons. 

5. Russell, Volcanoes of North America, Ginn & Co. 

6. Russell, North America, D. Appleton & Co. 

7. Hall, Geography of Minnesota, The H. W. Wilson Co. 

8. The American Deserts. National Geographic Magazine, 

April, 1904. 

9. Mill, The International Geography, D. Appleton & Co. 
10. Simonds, Geography of Texas, Ginn & Co. 



APPENDIX I 



473 









Surface 
Gravity 
Earth 

asl 


CO 


1 


CO 


00 


• 


OO 


CD 


00 

tH 


o 

05 


00 




^r 


o 


o 


rH 


o 


(M 


tH 


o 


o 



ai 3 cc * >> 



o) o 



« is M 






q;^ 










lO 


CO 



C<) 

o 




1-i 

T— 1 



lo 


CD 


CD 


00 


CO 


CD 



OS 


CD 


1— 1 


lO 


00 


00 


t- 


CD 


r-l 


o 


O 


O 


1— 1 


CO 


CD 



^ r-i 





lO 


00 


c^ 


OO 


Cvl 



OO OS 



O O i>- 

CO O iH 

O t-^ Ci^ 

co" t-" t-" 



o 


o 


o 


o 


o 


o 


o 


o 


iO 


o 


Oi 


OO 










CD 


CO 


rH 


^^ 


00 


t- 


CO 


CO 



s « 

o u 
o a 






m 



P ^ 



APPENDIX II 

SOME OF THE COMMON METHODS OF MAP 
PROJECTION 



The cylindrical projection supposes a cylinder of paper 
around the globe touching the equator and hence parallel 
to the axis of the globe. On this the meridians and paral- 
lels are projected at right angles to each other, the mer- 
idians forming vertical lines and the parallels horizontal 
ones. The meridians are equally spaced and all points on 
the equator are in their true proportion, but toward the 
poles the areas are much out of proportion. If the prd- 




FiG. 313. Cylindrical projection. 

jection of the parallels is from the center of the globe, the 
pole of the earth is at an infinite distance and cannot be 
represented ; also the polar regions are greatly exaggerated 
and not represented beyond 70 or 75 degrees. If the pro- 
jection of the parallels is at right angles to the axis, the 
polar regions are out of proportion in the opposite direc- 
tion. (See fig. 313.) 

Mercator^s projection is a modified form of the cylin- 

474 



METHODS OF MAP PROJECTION 



475 



drical, in which the parallels are so spaced that the degrees 
of latitude and longitude are in their proper proportions. 
It is much used by navigators in plotting the course at 
sea, because the directions are all true and the course can 
be plotted in a straight line. (Fig. 18, p. 34, is on Mer- 
cator's projection.) 

In the stereographic projection, commonly used in map- 
ping the hemispheres, a sheet of paper is placed without 
curving parallel to the axis of the globe, touching the 
equator at one point in the middle of the hemisphere to be 
mapped. The lines are then projected on the paper from 
the point at the opposite end of the diameter touched by 
the paper. 

The globular projection differs from the stereographic 
in being projected from a point 1.707 times the radius of 
the globe. 

The orthographic projection differs from the preceding 
in being projected from a point at infinity; that is, the 
lines of projection pass through the globe parallel to each 
other and normal to the paper. 







3 

k 










^ 


%K 




>^ 






\^^ 




/ 




/^ 


^ Globe. 


\____3^ 



Fig. 314. Conical projection. 



The conical projection assumes a cone touching the 
earth on the parallel passing through the middle of the 
area to be mapped and the lines projected on the cone from 



476 



PHYSICAL GEOGRAPHY 



the center of the globe. The cone is then split open on a 
meridian line and spread out flat. This is more accurate 
than any of the preceding for small areas away from the 
equator. In large areas the distortion becomes pronounced 
away from the center of the map ; but where greater accu- 
racy is required this defect is sometimes remedied in part 
by using a polyconic projection. 




Fia. 315. Polar projection. 



In the polar projection, as shown in fig. 315, the paper 
is placed tangent to the pole and from the center of the 
globe, one point on each parallel is projected to the paper 
as at P, Q, R. With N (the pole) as a center, circles are 



METHODS OF MAP PROJECTION 477 

drawn through these points for the parallels. Radial lines 
from the center (N) form the meridians. 

Polar 




Fig. 316. Projections illustrated with 
wire screen. 



In none of the above projections is a globe used in 
actual construction, but the lines are located by com- 
putation. 



GENERAL REFERENCES 

Special references are given at the end of each chapter. 
The following general reference books contain valuable 
data on different phases of the subject and should be con- 
sulted as far as possible by both teacher and students: 

1. Physiography by R. D. Salisbury, Henry Holt & Co. 

2. Text Book on Geology, 3 Vols., by Chamberlin and Salis- 
bury, Henry Holt & Co. 

3. Other text books on Physical Geography and Geology. 

4. The International Geography by 70 authors, D. Appleton 
& Co. 

5. Proceedings of the 8th International Geographical Con- 
gress, Washington, 1904. 

6'. Publications of the U. S. Geological Survey consisting of 
Geologic and Topographic Atlas, Monographs, Bulletins, Profes- 
sional Papers, Annual Reports, and Water Supply and Irrigation 
Papers. 

The contour maps of the Topographic Atlas are of special 
importance. They can be obtained from the Director of the U. S, 
Geological Survey, Washington, D. C, at five cents each or three 
dollars per hundred. 

PERIODICALS 

The National Geographic Magazine, Washington, D. C. 

The Journal of Geography, N. Y. 

The Bulletin of the American Geographical Society, N. Y. 

School Science and Mathematics, Chicago, 111. 

The Journal of Geology, Chicago, 111. 



478 



INDEX 



Abyssal life, 437 

Adirondack Mountains,457 

Aggrading, 71 

Aletsch Glacier, 139 

Alkali plains, 317 

Alkaline lakes, 113 

Alleghany plateau, 93, 327, 454 

Alluvial cone, 87 

fan, 86 

plain, 313 

soil, 263, 269 
Alpine glaciers, 141 
Aluminium ores, 250 
American Museum of Natural History, 

11, 282, 283, 284 
Andromeda nebula, 14 
Anemometer, 379 
Aneroid barometer, 353 
Antecedent river, 94 
Anticline, 337 
Aphelion, 22 
Aquifer, 42, 55, 56 
Arid climate, 97, 98 
Arroyo, 98, 330, 331 
Artesian well, 55, 311 
Atmosphere, 348 
Atoll, 222 
Augite, 244 
Aurora borealis, 396 
Ausable Chasm, 73, 322 

Bad Lands, 65, 330, 461 

Barnett Falls, 131 

Barogram, 354 

Barograph, 355 

Barometer, 352 

Barriers, 212, 344, 417, 431 

Bars, 209 

Base level, 71 

Bates' Hole, 65 

Beach, 208 

Beaver lakes, 105 

Biela's comet, 10 

Big trees, 412, 429 



Black Hills, 460 
Blind fish, 43 

Block mountains, 341, 470 
Bogs, 124 
Bore, tidal, 183 
Boston Mountains, 459 
Boulder clay, 148, 149 
Boulders, 158, 160, 161 
Breakers, 177 
Breakwater, 233 
Breccia, 257 
Building stone, 346 
Buttes, 323, 324, 462 

Calcite, 244 . 

Calderas, 295 

Calendar, 29 

Calories, 357 

Canoe mountains, 339, 340, 454 

Canyons, 322 

Caroline Bridge, 48 

Catskill Mountains, 455 

Cave deposits, 50 

Caves, 42 

Chamberlin, T, C, 13 

Charleston earthquake, 300 

Chimney rocks, 204 

Chinook, 377 

Cirques, 140 

Clay, 257 

Cliff glaciers, 141 

Climate, 288, 394, 400, 441 

Cloudburst, 387 

Clouds, 363, 382-385 

Coal, 258 

Coastal plain, 310, 312, 313, 355 

Cold wave, 377 

Colorado plateau, 467 

Composition of earth's crust, 238 

of atmosphere, 349 
Continental glaciers, 141 

shelf, 169 
Continents, 236 
Contour maps, 36 



479 



480 



INDEX 



Copper ores, 249 
Coral, 218 
Coal Creek, 80 
Coral reefs, 221 

harbors, 232 
Corrasion, 73, 152 
Crater Lake, 103 
Creep, 263, 264 
Crevasse, river, 82 

glacial, 145 
Crouse Boulder, 158 
Crystals, 241 
Cuesta, 463 
Cycle of erosion, 87 
Cyclones, 370, 371, 386, 889 
Cypress, 402 

Daniel's comet, 10 

Day, 30, 31 

Deeps, 174 

Deep sea deposits, 189, 190 

life, 194, 436, 437 

oozes, 190 
Degrading by streams, 70 
Deltas, 84, 101, 114, 460 
Delta harbors, 230^^ 
Desert plants, 407 
Deserts, 328-335, 419 
Dew, 381 
Dew polnr ^.SO 
Diastrophism, 274 
Diatoms, 117, 118, 119, 190 
Dikes, 205, 297 
Diorite, 260 
Directions, 20 
Disintegration of rock, 262 
Distributaries, 84, 314 
Divides, migration of, 95 
Dolomite, 245 
Domed mountains, 340 
Dredges, 172, 173 
Drift, glacial, 149 

oceanic, 187 
Drumlin, 150 
Dust, 351 

Earth, a magnet, 33 
motions of, 19 
origin of, 11 , 

part of solar system, 2 
revolution of, 21 



rota,tion of, 20 , 

shape of, 16, 17 

size of, 17 

structure of, 19 

shine, 5 
Earthquake waves, 178 
Earthquakes, 104, 299-307 
Eel grass, 218 
Eclipses, 7 

Economic features of coastal plains 
313 

of glaciers, 167 

of harbors, 232 

of mountains, 345 

of the ocean, 195 

of plateaus, 325 

of swamps and marshes, 125 
Ellipse, 9 

Engrafted rivers, 94 
Epeirogenic movement, 275 
Epiphytes, 403 
Eratosthenes, 18 
Esker, 150 
Eskimo, 443 
iiiureka Springs, 59 

Fall line, 135 

Falls, 125 

Faults, 322, 325 

Feldspar, 242 

Ferrel's law, 187 

Fiord harbors, 231 

Fissures, 296 

Flood plains, 78, 123, 315, 461 

Fluorite, 254 

Forests, 423-430 

Fossil, shore lines, 224 

lakes, 116 

reefs, 222 
Foucault's pendulum, 20 
Frost, 381 

Gabbro, 260 

Garden of the Gods, 160, 466 

Gegenshein, 398 

Geographic cycle, 270 

Geysers, 61 

Glacial channels, 162, 163 

plains, 317 

soils, 269 
Glaciers, 138-167 



INDEX 



481 



Glaciers, economic effects of, 167 
movements of, 143-164 
North American, 165 

Glauconite, 191 

Graded streams, 71 

Grand Canyon, 323, 468, 469 

Granite, 259, 260 

Graphite, 253 

Gravitation, 17 

Great Interior Basin, 112 

Great Lakes, 111 

Great Salt Lake, 108, 112, 214, 225 

Great Valley, 248, 444, 454 

Groundwater, 41, 53 

Gulf Stream, 185 

Gypsum, 252 

Halite, 250 
Halley's comet, 10 
Hanging valleys, 156, 157 
Harbors, 228-234 
Hardness, scale of, 240 
Hematite, 246 
Hog Back Mountains, 465 
Hook, 209 . 
Hornblende, 244 
Horse latitude, 370 
Hot springs, 61 
Humidity, 379 
Hurricane, 297 
Hygrodeik, 381 
Hygrometer, 380-381 
Hyperbola, 9 

Icebergs, 160 

Ice tables and pinnacles, 146 

Indus River, 85 

Insolation, 6 

International date line, 31 

Iron ores, 246 

Iroquois Lake, 116 

Islands, 236 

Isobars, 356 

Isoclinal lines, 33 

Isogonic lines, 33 

Isostacy, 277 

Isotherms, 364 

Kame, 149 
Kaolin, 245 
Karsten, 46 



Kettle holes, 149 
Kingston earthquake, 305 

Laccolites, 297, 341 
Lacustrine plains, 316 
Lakes, 100-137, 224 

disappearance of, 114 
function of, 119 
in arid regions, 121 
levels, 113 
origin of, 100 
shores, 224 
Land, 235 
Latitude, 24, 25 
Lava, 289 
Lead ores, 249 
Levee, 82 
Levee lakes, 84 
Life history of lakes, 120 
of a land area, 270 
of mountains, 342 
of a river, 87 

of sedimentary rocks, 269 
of a volcano, 296 
Life in caves, 43 

in lakes and rivers, 117 
Life zones, 414 
Lighter, 233 
Lightning, 395 
Limestone, 223, 258 
Limonite, 246 
Lisbon earthquake, 306 
Longitude, 24, 27, 28 
Lost River, 45 

Magnesite, 254 

Magnetism, 32 

Magnetite, 248 

Mammoth Cave, 44 

Man, distribution of, 439 

Mangrove, 217 

Mantle rock, 264 

Maps, 35, 36, 474 

Marble, 258, 261 

Marengo Cave, 51 

Marl, 114 

Marshes, 123 

Maturity of topography, 271 

Meanders, 79, 80 

Mediterranean seas, 170 

Mercator's projection, 474 



482 



INDEX 



Mesa, 323 

Metamorphic rocks, 260 

Meteorites, 10 

Meteors, 10 

Mica, 243 

Minerals, 239 

Mineral springs, 60 

Mirage, 397 

Mississippi River, 458, 81, 83, 84, 

315 
Mississippi valley earthquake, 299 
Missouri River, 77 
Monadnocks, 271, 272, 318 
Monsoon, 377 
Moraines, 147 
Mount Pelee, 281 
Mount Potosi, 67 
Mount Vesuvius, 278, 279 
Mountains, 262, 335, 346, 419, 442 
Muck, 268 
Muirs Butte, 292 

Nadir, 21 

Natural Bridge, 46, 206, 207 

Nebula, 12, 13 

Nebular hypothesis, 12 

Neve, 140 

Niagara Falls, 126, 127 

Northeaster, 377 

North Platte River, 88 

Obsidian, 260 

Ocean, 169 

Ocean life in, 192, 194 

currents, 185 
Onyx marble, 51 
Oozes, 190 
Ores, 246 

Orogenic movement, 275, 336 
Ouray, Colo., 86, 142, 153, 154, 155 
Overloaded stream, 76, 77 
Ox-bow lakes, 81 
Oxygen, 349 

Parabola, 9 

Peat, 115 

Pelagic life, 194, 438 

Peneplain, 271, 318 

Perched boulders, 158 

Perihelion, 22 

Phases of moon, 5 



Physical Geography, 1 
Physiographic agencies, 274 

features, 309 

regions, 449 
Piedmont glaciers, 141 

plateau, 452 
Pigmies, 443 
Pilaster, 51 
Pittsburg, 444, 445 
Plains, 309, 419 
Planetoids, 2, 3 
Planets, 2 

Planetesimal hypothesis, 13 
Plant Geography, 402 
Plateaus, 321 
Playas, 98 

Polar projection, 476 
Pot-hole, 73, 155 
Potomac River, 70 
Prairies, 320, 411 
Precipitation, 385 
Projections, 35, 474 
Pumice, 260 
Pyrite, 248 

Quaking bogs, 123 
Quartz, 240 
Quartzite, 261 

Rainbow, 397 
Rainfall, 40, 386 
Rain gauge, 385 
Rapids, 69 
Reaches, 69, 134 
Reelsfoot Lake, 104 
References, 39, 99, 137, 167, 196, 
234, 273, 307, 346, 398, 447, 471, 
478 
Residual soil, 267, 268 
Reversed drainage, 93 

rivers, 92 
Revolution of the earth, 21 
River deposits, 77 

profile, 68, 69 

piracy, 95 

swamp, 83 

valley, 64 
Rivers, 40, 63, 67, 72 
Rocking stones, 158 
Rocks, 254 
Rocky Mountains, 464 



INDEX 



483 



Saint Elmo's fire, 396 

Salinas, 112 

Salt, 250 

Salt lakes. 111, 214 

marshes, 124 

plains, 317 
Salts of the ocean, 171 
Salton sink, 107, 108 
Sand dunes, 226 
Sandstone, 256 

San Francisco earthquake, 302-305 
Sargasso seas, 186 
Sargassum, 407 
Satellite, 4 
Sea caves, 206 
Sea water, composition of, 170 

density of, 172 

depth of, 174 

temperature of, 175 
Seasons, 22 

Sedimentary rocks, 255 
Seepage, 60 
Seiche, 114 
Shale, 257 
Shooting stars, 10 
Shore cliff, 203 

lines, 197-234, 277 

terraces, 213 
Sink holes, 45, 106 
Snow fields, 138 
Soil, 262, 402 
Solar day, 30 

system, 2, 3, 473 

time, 30 
Sounding and dredging, 172 
Spit, 209 

Spouting caves, 208 
Springs, 51, 58 
Stalactite, 50 
Stalagmite, 50 
Standard time, 31 
Subsequent streams, 94 
Sulphur, 252 
Superimposed rivers, 92 
Swamps, 123 
Syenite, 260 
Syncline, 337 

Talc, 253 
Talus cone, 87 
Taughannock Creek, 78 



Temperature, 357, 360, 413 

Tent meteorite, 11 

Terminator, 5 

Terraced mountains, 338 

Terraces, 92, 93, 123 

Thermogram, 358 

Thermograph, 359 

Thermometer, 357 

Thunderstorm, 387 

Tides, 181, 184 

Till, 149 

Timber line, 4 Iff 

Time, 29 

Tinker's Falls, 134 

Toad Stool Park, 66 

Topogi;^phic atlas, 38, 478 

Topography of ocean bottom, 188, 238 

of shore lines, 197, 203 
Tornado, 375, 376 
Transportation by rivers, 74, 75 
Trade wind, 368 
Travertine, 51 
Tripoli, 118 
Tufa, 51, 290 
Ttindras, 321 

Underloaded stream, 76 
Undertow, 178 
Uncompaghre Creek, 76 
Universe, 3 

Vegetation, 402 

Vegetation on the shore line, 217 
Veins, 48, 49 
Volcanic harbor, 231 
mountains, 341 
Volcano, 279, 287, 295 

Wadies, 97 

Water, • effect on life, 403 

Water gaps, 94 

plants, 404 

table, 41, 54 

zone, 41 
Waterspout, 376 
Watkins Glen, 73 
Waves, 176, 179, 181, 199, 200 
Weather, 388 
Weather forecast, 393 

map, 390 



484 



INDEX 



Wells, artesian, 55 

common, 54 
Whistling caves, 208 
Willamette meteorite, 11 
Wind, 366, 368-379 
Wind gaps, 94 
Work of rivers, 72 



Wyandotte Cave, 52 

Youth of a river, 87 

Zinc ores, 259 

Zodiacal light, 398 

Zones, 366, 367, 393, 413 

Zoological provinces, 430, 431, 432 



' OFTHt 

UNIVERSITY 

OF 

califoi 



UNIVEKSITY OF CAI.IFOKNIA LIUKAUY 



THIS BOOK IS DUE ON THE LAST DATE 
STAMPED BELOW 



JUL 



(4 



im \ 8 t514i 



m 



L se tsm 






JUL 31 1915 
AUG 18 1915 

AUG 28 19V 
DEC 26 1924 

DEC 2S 



JUw'67CRP 

REC'D LO 

JAN : 



•*> 



307n-6,'14 



m^ ;'s? 







A 



188500